diff --git a/ExpressionEngine/ExpressionGrammar.cs b/ExpressionEngine/ExpressionGrammar.cs
index b58a265..99e5fa7 100644
--- a/ExpressionEngine/ExpressionGrammar.cs
+++ b/ExpressionEngine/ExpressionGrammar.cs
@@ -10,11 +10,14 @@ namespace ExpressionEngine
 {
     public class ExpressionGrammar
     {
+        private readonly IList<IFunctionDefinition> _functionDefinitions;
         private readonly Parser<IRule> _method;
         private readonly Parser<Task<ValueContainer>> _input;
 
-        public ExpressionGrammar(IEnumerable<IFunction> functions)
+        public ExpressionGrammar(IEnumerable<IFunction> functions, IEnumerable<IFunctionDefinition> functionDefinitions)
         {
+            _functionDefinitions = functionDefinitions?.ToList();
+
             var functionCollection = functions ?? throw new ArgumentNullException(nameof(functions));
 
             #region BasicAuxParsers
@@ -124,13 +127,25 @@ from t in simpleString.Or(allowedCharacters).Many()
 
         public async ValueTask<string> EvaluateToString(string input)
         {
-            var output = await _input.Parse(input);
+            var output = await PreAnalyzeAndParse(input);
 
             return output.GetValue<string>();
         }
 
         public async ValueTask<ValueContainer> EvaluateToValueContainer(string input)
         {
+            return await PreAnalyzeAndParse(input);
+        }
+
+        private async ValueTask<ValueContainer> PreAnalyzeAndParse(string input)
+        {
+            if (_functionDefinitions != null)
+            {
+                input = _functionDefinitions.Aggregate(input,
+                    (current, functionDefinition) =>
+                        current.Replace(functionDefinition.From, functionDefinition.To));
+            }
+
             return await _input.Parse(input);
         }
     }
diff --git a/ExpressionEngine/FlowRunnerDependencyExtension.cs b/ExpressionEngine/FlowRunnerDependencyExtension.cs
index 21b0e4b..41fba1b 100644
--- a/ExpressionEngine/FlowRunnerDependencyExtension.cs
+++ b/ExpressionEngine/FlowRunnerDependencyExtension.cs
@@ -1,4 +1,5 @@
 using ExpressionEngine.Functions.Base;
+using ExpressionEngine.Functions.CustomException;
 using ExpressionEngine.Functions.Implementations.CollectionFunctions;
 using ExpressionEngine.Functions.Implementations.ConversionFunctions;
 using ExpressionEngine.Functions.Implementations.LogicalComparisonFunctions;
@@ -25,6 +26,22 @@ public static void AddExpressionEngine(this IServiceCollection services)
             services.AddTransient<IFunction, GreaterFunction>();
         }
 
+        /// <summary>
+        /// Added FunctionDefinition to service collection.
+        /// </summary>
+        /// <param name="services"></param>
+        /// <param name="fromFunctionName">The name of the function, without function parenthesis</param>
+        /// <param name="toExpression">The full expression which is inserted</param>
+        public static void AddFunctionDefinition(this IServiceCollection services, string fromFunctionName, string toExpression)
+        {
+            if (fromFunctionName.EndsWith("()"))
+            {
+                throw new ArgumentError($"{nameof(fromFunctionName)} cannot end in ()");
+            }
+
+            services.AddSingleton<IFunctionDefinition>(new FunctionDefinition{From = fromFunctionName + "()", To = toExpression});
+        }
+
         private static void AddStringFunctions(IServiceCollection services)
         {
             services.AddTransient<IFunction, ConcatFunction>();
diff --git a/ExpressionEngine/FunctionDefinition.cs b/ExpressionEngine/FunctionDefinition.cs
new file mode 100644
index 0000000..7d92b49
--- /dev/null
+++ b/ExpressionEngine/FunctionDefinition.cs
@@ -0,0 +1,18 @@
+namespace ExpressionEngine
+{
+    /// <summary>
+    /// A Function Definition is a map to a collected set of functions.
+    /// - currentCellValue() -> formvalue(logicalName())[idx()][attributeLogicalName()]
+    /// </summary>
+    public class FunctionDefinition : IFunctionDefinition
+    {
+        /// <summary>
+        /// The from 'function name' which is replaced
+        /// </summary>
+        public string From { get; set; }
+        /// <summary>
+        /// The replacement
+        /// </summary>
+        public string To { get; set; }
+    }
+}
\ No newline at end of file
diff --git a/ExpressionEngine/Functions/Implementations/StringFunctions/ConcatFunction.cs b/ExpressionEngine/Functions/Implementations/StringFunctions/ConcatFunction.cs
index b2db84d..7cf7157 100644
--- a/ExpressionEngine/Functions/Implementations/StringFunctions/ConcatFunction.cs
+++ b/ExpressionEngine/Functions/Implementations/StringFunctions/ConcatFunction.cs
@@ -34,7 +34,7 @@ public override ValueTask<ValueContainer> ExecuteFunction(params ValueContainer[
             }
 
             return new ValueTask<ValueContainer>(new ValueContainer(parameters.Aggregate("",
-                (current, value) => current + AuxiliaryMethods.VcIsString(value))));
+                (current, value) => current + value)));
         }
     }
 }
\ No newline at end of file
diff --git a/ExpressionEngine/IFunctionDefinition.cs b/ExpressionEngine/IFunctionDefinition.cs
new file mode 100644
index 0000000..d95c7d5
--- /dev/null
+++ b/ExpressionEngine/IFunctionDefinition.cs
@@ -0,0 +1,18 @@
+namespace ExpressionEngine
+{
+    /// <summary>
+    /// Interface to function definitions
+    /// </summary>
+    public interface IFunctionDefinition
+    {
+        /// <summary>
+        /// The name of the 'function' which is replaced
+        /// </summary>
+        string From { get; set; }
+        
+        /// <summary>
+        /// What it is replaced with 
+        /// </summary>
+        string To { get; set; }
+    }
+}
\ No newline at end of file
diff --git a/Test/ExpressionGrammarTest.cs b/Test/ExpressionGrammarTest.cs
index bb14d9f..599278a 100644
--- a/Test/ExpressionGrammarTest.cs
+++ b/Test/ExpressionGrammarTest.cs
@@ -20,7 +20,7 @@ public void SetUp()
 
             var functions = new List<IFunction> {_dummyFunction};
 
-            _expressionGrammar = new ExpressionGrammar(functions);
+            _expressionGrammar = new ExpressionGrammar(functions, null);
         }
 
         [Test]
diff --git a/Test/FunctionDefinitionTests.cs b/Test/FunctionDefinitionTests.cs
new file mode 100644
index 0000000..07b4989
--- /dev/null
+++ b/Test/FunctionDefinitionTests.cs
@@ -0,0 +1,47 @@
+using System;
+using System.Threading.Tasks;
+using ExpressionEngine;
+using ExpressionEngine.Functions.CustomException;
+using Microsoft.Extensions.DependencyInjection;
+using NUnit.Framework;
+
+namespace Test
+{
+    public class FunctionDefinitionTests
+    {
+        private ServiceCollection _serviceCollection;
+
+        [SetUp]
+        public void Setup()
+        {
+            _serviceCollection = new ServiceCollection();
+            _serviceCollection.AddExpressionEngine();
+            _serviceCollection.AddFunctionDefinition("addAndConcat", "concat('result of 1+1 is: ', add(1,1))");
+        }
+
+        [TestCase]
+        public async Task TestFunctionDef()
+        {
+            var ee = _serviceCollection.BuildServiceProvider().GetRequiredService<IExpressionEngine>();
+            const string expectedResult = "result of 1+1 is: 2!";
+
+            var actualResult = await ee.Parse("@concat(addAndConcat(), '!')");
+
+            Assert.AreEqual(expectedResult, actualResult);
+        }
+
+        [TestCase]
+        public void TestFunctionException()
+        {
+            const string expectedMessage = "fromFunctionName cannot end in ()";
+            
+            var exception = Assert.Throws<ArgumentError>(() =>
+            {
+                _serviceCollection.AddFunctionDefinition("addAndConcat()", "concat('result of 1+1 is: ', add(1,1))");
+            });
+            
+            Assert.NotNull(exception);
+            Assert.AreEqual(expectedMessage,exception.Message);
+        }
+    }
+}
\ No newline at end of file
diff --git a/Test/NullCoalescingTest2.cs b/Test/NullCoalescingTest2.cs
index 22a7429..fc7339c 100644
--- a/Test/NullCoalescingTest2.cs
+++ b/Test/NullCoalescingTest2.cs
@@ -19,7 +19,7 @@ public void Setup()
 
             var functions = new List<IFunction> {_returnData};
 
-            _expressionGrammar = new ExpressionGrammar(functions);
+            _expressionGrammar = new ExpressionGrammar(functions, null);
         }
 
         [Test]