CTS: Common Type System : It defines a set of rules how datatypes are declared, used and managed in the run time. It facilitates cross language integration ,type safety and code performance. CLS: Common Language Specifications: It defines a set of rules and restrictions that every language must follow that runs under .NET environment. it is useful for cross language integration. CTS Example: C# integer Int & VB.NET integer are compiled to MSIL as Int32 CLS Example: C# ; is the statement terminator but not in VB.NET in C# method add() & Add() are different but not in VB.NET