diff --git a/core/modules/filters.js b/core/modules/filters.js
index e0a2779e3..5f4eb6492 100644
--- a/core/modules/filters.js
+++ b/core/modules/filters.js
@@ -244,13 +244,15 @@ exports.compileFilter = function(filterString) {
 				var operands = [],
 					operatorFunction;
 				if(!operator.operator) {
+					// Use the "title" operator if no operator is specified
 					operatorFunction = filterOperators.title;
 				} else if(!filterOperators[operator.operator]) {
-					operatorFunction = filterOperators.field;
+					// Unknown operators treated as "unknown" - at run time we can distinguish between a custom operator and falling back to the default "field" operator
+					operatorFunction = filterOperators.unknown;
 				} else {
+					// Use the operator function
 					operatorFunction = filterOperators[operator.operator];
 				}
-
 				$tw.utils.each(operator.operands,function(operand) {
 					if(operand.indirect) {
 						operand.value = self.getTextReference(operand.text,"",currTiddlerTitle);
diff --git a/core/modules/filters/unknown.js b/core/modules/filters/unknown.js
new file mode 100644
index 000000000..93b42c706
--- /dev/null
+++ b/core/modules/filters/unknown.js
@@ -0,0 +1,59 @@
+/*\
+title: $:/core/modules/filters/unknown.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator for handling unknown filter operators
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var fieldFilterOperatorFn = require("$:/core/modules/filters/field.js").field;
+
+/*
+Export our filter function
+*/
+exports.unknown = function(source,operator,options) {
+	var customDefinitionTitle = "[" + operator.operator + "[]]",
+		customDefinition = options.widget && options.widget.getVariableInfo && options.widget.getVariableInfo(customDefinitionTitle);
+	if(customDefinition && customDefinition.srcVariable) {
+		var variables = Object.create(null);
+		$tw.utils.each(customDefinition.srcVariable.variableParams,function(param,index) {
+			var value = operator.operands[index];
+			if(value === undefined) {
+				value = param["default"] || "";
+			}
+			variables[param.name] = value;
+		});
+		var getVariable = function(name,opts) {
+			if(name in variables) {
+				return variables[name];
+			} else {
+				return options.widget.getVariable(name,opts);
+			};
+		};
+		var getVariableInfo = function(name,opts) {
+			return options.widget.getVariableInfo(name,opts);
+		}
+		var list = options.wiki.filterTiddlers(customDefinition.srcVariable.value,{getVariable: getVariable,getVariableInfo: getVariableInfo},source);
+		if(operator.prefix === "!") {
+			var results = [];
+			source(function(tiddler,title) {
+				if(list.indexOf(title) === -1) {
+					results.push(title);
+				}
+			});
+			return results;
+		} else {
+			return list;
+		}
+	} else {
+		return fieldFilterOperatorFn(source,operator,options);
+	}
+};
+
+})();
diff --git a/editions/test/tiddlers/tests/data/custom-operators/NestedParameterised.tid b/editions/test/tiddlers/tests/data/custom-operators/NestedParameterised.tid
new file mode 100644
index 000000000..cbeb1570d
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/custom-operators/NestedParameterised.tid
@@ -0,0 +1,24 @@
+title: CustomOperators/NestedParameterised
+description: Nested parameterised custom operator usage
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+\function [dividebysomething[]](first:ignored,factor:0.5)
+[divide[2]multiply<factor>]
+\end
+
+\function [multiplebysomething[]](first:ignored,factor:2)
+[multiply[2]dividebysomething[],<factor>]
+\end
+
+<$text text={{{ [[123]multiplebysomething[]] }}}/>
+-
+<$text text={{{ [[123]multiplebysomething[x],[4]] }}}/>
+
++
+title: ExpectedResult
+
+<p>246-492</p>
\ No newline at end of file
diff --git a/editions/test/tiddlers/tests/data/custom-operators/Parameterised.tid b/editions/test/tiddlers/tests/data/custom-operators/Parameterised.tid
new file mode 100644
index 000000000..42ca40ec6
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/custom-operators/Parameterised.tid
@@ -0,0 +1,20 @@
+title: CustomOperators/Parameterised
+description: Parameterised custom operator usage
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+\function [multiplybysomething[]](first:ignored,factor:2)
+[multiply[2]multiply<factor>]
+\end
+
+<$text text={{{ [[123]multiplybysomething[]] }}}/>
+-
+<$text text={{{ [[123]multiplybysomething[x],[4]] }}}/>
+
++
+title: ExpectedResult
+
+<p>492-984</p>
\ No newline at end of file
diff --git a/editions/test/tiddlers/tests/data/custom-operators/Simple.tid b/editions/test/tiddlers/tests/data/custom-operators/Simple.tid
new file mode 100644
index 000000000..076a54dad
--- /dev/null
+++ b/editions/test/tiddlers/tests/data/custom-operators/Simple.tid
@@ -0,0 +1,16 @@
+title: CustomOperators/Simple
+description: Simple custom operator usage
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\function [multiplybytwo[]]()
+[multiply[2]]
+\end
+
+<$text text={{{ [[123]multiplybytwo[]] }}}/>
++
+title: ExpectedResult
+
+<p>246</p>
\ No newline at end of file