--- a/runtime/doc/tags
+++ b/runtime/doc/tags
@@ -5850,6 +5850,7 @@ abandon	editing.txt	/*abandon*
 abbreviations	map.txt	/*abbreviations*
 abel.vim	syntax.txt	/*abel.vim*
 abs()	builtin.txt	/*abs()*
+abstract-method	vim9class.txt	/*abstract-method*
 acos()	builtin.txt	/*acos()*
 active-buffer	windows.txt	/*active-buffer*
 ada#Create_Tags()	ft_ada.txt	/*ada#Create_Tags()*

--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -358,6 +358,16 @@ class, for which objects can be created.
 An abstract class is defined the same way as a normal class, except that it
 does not have any new() method. *E1359*
 
+						*abstract-method*
+An abstract method can be defined in an abstract class by using the "abstract"
+prefix when defining the function: >
+
+	abstract class Shape
+	   abstract def Draw()
+	endclass
+
+A class extending the abstract class must implement all the abstract methods.
+Class methods in an abstract class can also be abstract methods.
 
 ==============================================================================
 

--- a/src/errors.h
+++ b/src/errors.h
@@ -3495,6 +3495,12 @@ EXTERN char e_duplicate_member_str[]
 	INIT(= N_("E1369: Duplicate member: %s"));
 EXTERN char e_cannot_define_new_function_as_static[]
 	INIT(= N_("E1370: Cannot define a \"new\" function as static"));
+EXTERN char e_abstract_must_be_followed_by_def_or_static[]
+	INIT(= N_("E1371: Abstract must be followed by \"def\" or \"static\""));
+EXTERN char e_abstract_method_in_concrete_class[]
+	INIT(= N_("E1372: Abstract method \"%s\" cannot be defined in a concrete class"));
+EXTERN char e_abstract_method_str_not_found[]
+	INIT(= N_("E1373: Abstract method \"%s\" is not implemented"));
 EXTERN char e_cannot_mix_positional_and_non_positional_str[]
 	INIT(= N_("E1400: Cannot mix positional and non-positional arguments: %s"));
 EXTERN char e_fmt_arg_nr_unused_str[]

--- a/src/structs.h
+++ b/src/structs.h
@@ -1515,6 +1515,7 @@ struct itf2class_S {
 
 #define CLASS_INTERFACE	    1
 #define CLASS_EXTENDED	    2	    // another class extends this one
+#define CLASS_ABSTRACT	    4	    // abstract class
 
 // "class_T": used for v_class of typval of VAR_CLASS
 // Also used for an interface (class_flags has CLASS_INTERFACE).
@@ -1875,6 +1876,7 @@ struct ufunc_S
 
 #define FC_OBJECT   0x4000	// object method
 #define FC_NEW	    0x8000	// constructor
+#define FC_ABSTRACT 0x10000	// abstract method
 
 #define MAX_FUNC_ARGS	20	// maximum number of function arguments
 #define VAR_SHORT_LEN	20	// short variable name length

--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -4473,4 +4473,140 @@ enddef
 "   v9.CheckScriptSuccess(lines)
 " enddef
 
+" Test for abstract methods
+def Test_abstract_method()
+  # Use two abstract methods
+  var lines =<< trim END
+    vim9script
+    abstract class A
+      def M1(): number
+        return 10
+      enddef
+      abstract def M2(): number
+      abstract def M3(): number
+    endclass
+    class B extends A
+      def M2(): number
+        return 20
+      enddef
+      def M3(): number
+        return 30
+      enddef
+    endclass
+    var b = B.new()
+    assert_equal([10, 20, 30], [b.M1(), b.M2(), b.M3()])
+  END
+  v9.CheckScriptSuccess(lines)
+
+  # Don't define an abstract method
+  lines =<< trim END
+    vim9script
+    abstract class A
+      abstract def Foo()
+    endclass
+    class B extends A
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1373: Abstract method "Foo" is not implemented')
+
+  # Use abstract method in a concrete class
+  lines =<< trim END
+    vim9script
+    class A
+      abstract def Foo()
+    endclass
+    class B extends A
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1372: Abstract method "abstract def Foo()" cannot be defined in a concrete class')
+
+  # Use abstract method in an interface
+  lines =<< trim END
+    vim9script
+    interface A
+      abstract def Foo()
+    endinterface
+    class B implements A
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1372: Abstract method "abstract def Foo()" cannot be defined in a concrete class')
+
+  # Abbreviate the "abstract" keyword
+  lines =<< trim END
+    vim9script
+    class A
+      abs def Foo()
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1065: Command cannot be shortened: abs def Foo()')
+
+  # Use "abstract" with a member variable
+  lines =<< trim END
+    vim9script
+    abstract class A
+      abstract this.val = 10
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1371: Abstract must be followed by "def" or "static"')
+
+  # Use a static abstract method
+  lines =<< trim END
+    vim9script
+    abstract class A
+      abstract static def Foo(): number
+    endclass
+    class B extends A
+      static def Foo(): number
+        return 4
+      enddef
+    endclass
+    assert_equal(4, B.Foo())
+  END
+  v9.CheckScriptSuccess(lines)
+
+  # Type mismatch between abstract method and concrete method
+  lines =<< trim END
+    vim9script
+    abstract class A
+      abstract def Foo(a: string, b: number): list<number>
+    endclass
+    class B extends A
+      def Foo(a: number, b: string): list<string>
+        return []
+      enddef
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1407: Member "Foo": type mismatch, expected func(string, number): list<number> but got func(number, string): list<string>')
+
+  # Use an abstract class to invoke an abstract method
+  # FIXME: This should fail
+  lines =<< trim END
+    vim9script
+    abstract class A
+      abstract static def Foo()
+    endclass
+    A.Foo()
+  END
+  v9.CheckScriptSuccess(lines)
+
+  # Invoke an abstract method from a def function
+  lines =<< trim END
+    vim9script
+    abstract class A
+      abstract def Foo(): list<number>
+    endclass
+    class B extends A
+      def Foo(): list<number>
+        return [3, 5]
+      enddef
+    endclass
+    def Bar(c: B)
+      assert_equal([3, 5], c.Foo())
+    enddef
+    var b = B.new()
+    Bar(b)
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -5021,6 +5021,7 @@ define_function(
     // Do not define the function when getting the body fails and when
     // skipping.
     if (((class_flags & CF_INTERFACE) == 0
+		&& (class_flags & CF_ABSTRACT_METHOD) == 0
 		&& get_function_body(eap, &newlines, line_arg, lines_to_free)
 								       == FAIL)
 	    || eap->skip)

--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@ static char *(features[]) =
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1885,
+/**/
     1884,
 /**/
     1883,

--- a/src/vim.h
+++ b/src/vim.h
@@ -2915,5 +2915,6 @@ long elapsed(DWORD start_tick);
 // Flags used by "class_flags" of define_function()
 #define CF_CLASS	1	// inside a class
 #define CF_INTERFACE	2	// inside an interface
+#define CF_ABSTRACT_METHOD	4	// inside an abstract class
 
 #endif // VIM__H

--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -308,21 +308,19 @@ validate_extends_class(char_u *extends_n
 	semsg(_(e_class_name_not_found_str), extends_name);
 	return success;
     }
+
+    if (tv.v_type != VAR_CLASS
+	    || tv.vval.v_class == NULL
+	    || (tv.vval.v_class->class_flags & CLASS_INTERFACE) != 0)
+	semsg(_(e_cannot_extend_str), extends_name);
     else
     {
-	if (tv.v_type != VAR_CLASS
-		|| tv.vval.v_class == NULL
-		|| (tv.vval.v_class->class_flags & CLASS_INTERFACE) != 0)
-	    semsg(_(e_cannot_extend_str), extends_name);
-	else
-	{
-	    class_T *extends_cl = tv.vval.v_class;
-	    ++extends_cl->class_refcount;
-	    *extends_clp = extends_cl;
-	    success = TRUE;
-	}
-	clear_tv(&tv);
+	class_T *extends_cl = tv.vval.v_class;
+	++extends_cl->class_refcount;
+	*extends_clp = extends_cl;
+	success = TRUE;
     }
+    clear_tv(&tv);
 
     return success;
 }
@@ -392,6 +390,65 @@ validate_extends_members(
 }
 
 /*
+ * When extending an abstract class, check whether all the abstract methods in
+ * the parent class are implemented.  Returns TRUE if all the methods are
+ * implemented.
+ */
+    static int
+validate_extends_methods(
+    garray_T	*classmethods_gap,
+    garray_T	*objmethods_gap,
+    class_T	*extends_cl)
+{
+    for (int loop = 1; loop <= 2; ++loop)
+    {
+	// loop == 1: check class methods
+	// loop == 2: check object methods
+	int extends_method_count = loop == 1
+				? extends_cl->class_class_function_count
+				: extends_cl->class_obj_method_count;
+	if (extends_method_count == 0)
+	    continue;
+
+	ufunc_T **extends_methods = loop == 1
+				? extends_cl->class_class_functions
+				: extends_cl->class_obj_methods;
+
+	int method_count = loop == 1 ? classmethods_gap->ga_len
+						: objmethods_gap->ga_len;
+	ufunc_T **cl_fp = (ufunc_T **)(loop == 1
+						? classmethods_gap->ga_data
+						: objmethods_gap->ga_data);
+
+	for (int i = 0; i < extends_method_count; i++)
+	{
+	    ufunc_T *uf = extends_methods[i];
+	    if ((uf->uf_flags & FC_ABSTRACT) == 0)
+		continue;
+
+	    int method_found = FALSE;
+
+	    for (int j = 0; j < method_count; j++)
+	    {
+		if (STRCMP(uf->uf_name, cl_fp[j]->uf_name) == 0)
+		{
+		    method_found = TRUE;
+		    break;
+		}
+	    }
+
+	    if (!method_found)
+	    {
+		semsg(_(e_abstract_method_str_not_found), uf->uf_name);
+		return FALSE;
+	    }
+	}
+    }
+
+    return TRUE;
+}
+
+/*
  * Check the members of the interface class "ifcl" match the class members
  * ("classmembers_gap") and object members ("objmembers_gap") of a class.
  * Returns TRUE if the class and object member names are valid.
@@ -1266,6 +1323,31 @@ early_ret:
 	    }
 	}
 
+	int abstract_method = FALSE;
+	char_u *pa = p;
+	if (checkforcmd(&p, "abstract", 3))
+	{
+	    if (STRNCMP(pa, "abstract", 8) != 0)
+	    {
+		semsg(_(e_command_cannot_be_shortened_str), pa);
+		break;
+	    }
+
+	    if (!is_abstract)
+	    {
+		semsg(_(e_abstract_method_in_concrete_class), pa);
+		break;
+	    }
+
+	    abstract_method = TRUE;
+	    p = skipwhite(pa + 8);
+	    if (STRNCMP(p, "def", 3) != 0 && STRNCMP(p, "static", 6) != 0)
+	    {
+		emsg(_(e_abstract_must_be_followed_by_def_or_static));
+		break;
+	    }
+	}
+
 	int has_static = FALSE;
 	char_u *ps = p;
 	if (checkforcmd(&p, "static", 4))
@@ -1344,8 +1426,13 @@ early_ret:
 	    ea.cookie = eap->cookie;
 
 	    ga_init2(&lines_to_free, sizeof(char_u *), 50);
+	    int class_flags;
+	    if (is_class)
+		class_flags = abstract_method ? CF_ABSTRACT_METHOD : CF_CLASS;
+	    else
+		class_flags = CF_INTERFACE;
 	    ufunc_T *uf = define_function(&ea, NULL, &lines_to_free,
-					   is_class ? CF_CLASS : CF_INTERFACE);
+								class_flags);
 	    ga_clear_strings(&lines_to_free);
 
 	    if (uf != NULL)
@@ -1353,7 +1440,8 @@ early_ret:
 		char_u	*name = uf->uf_name;
 		int	is_new = STRNCMP(name, "new", 3) == 0;
 
-		if (is_new && !is_valid_constructor(uf, is_abstract, has_static))
+		if (is_new && !is_valid_constructor(uf, is_abstract,
+								has_static))
 		{
 		    func_clear_free(uf, FALSE);
 		    break;
@@ -1374,6 +1462,9 @@ early_ret:
 		    if (is_new)
 			uf->uf_flags |= FC_NEW;
 
+		    if (abstract_method)
+			uf->uf_flags |= FC_ABSTRACT;
+
 		    ((ufunc_T **)fgap->ga_data)[fgap->ga_len] = uf;
 		    ++fgap->ga_len;
 		}
@@ -1430,12 +1521,20 @@ early_ret:
 	success = validate_extends_class(extends, &extends_cl);
     VIM_CLEAR(extends);
 
-    // Check the new class members and object members doesn't duplicate the
+    // Check the new class members and object members are not duplicates of the
     // members in the extended class lineage.
     if (success && extends_cl != NULL)
 	success = validate_extends_members(&classmembers, &objmembers,
 								extends_cl);
 
+    // When extending an abstract class, make sure all the abstract methods in
+    // the parent class are implemented.  If the current class is an abstract
+    // class, then there is no need for this check.
+    if (success && !is_abstract && extends_cl != NULL
+				&& (extends_cl->class_flags & CLASS_ABSTRACT))
+	success = validate_extends_methods(&classfunctions, &objmethods,
+								extends_cl);
+
     class_T **intf_classes = NULL;
 
     // Check all "implements" entries are valid.
@@ -1463,6 +1562,8 @@ early_ret:
 	    goto cleanup;
 	if (!is_class)
 	    cl->class_flags = CLASS_INTERFACE;
+	else if (is_abstract)
+	    cl->class_flags = CLASS_ABSTRACT;
 
 	cl->class_refcount = 1;
 	cl->class_name = vim_strnsave(name_start, name_end - name_start);