SUMMARY This chapter began with a discussion of delegates and their use as references to methods or callbacks. It introduced a powerful concept for passing a set of instructions to call in a different location, rather than immediately, when the instructions are coded. Following on the heels of a brief look at the C# 2.0 concept of anonymous methods, the chapter introduced the C# 3.0 concept of lambda expressions, a syntax that supersedes (although doesn't eliminate) the C# 2.0 anonymous method syntax. Regardless of the syntax, these constructs allow programmers to assign a set of instructions to a variable directly, without defining an explicit method that contains the instructions. This provides significant flexibility for programming instructions dynamically within the method-a powerful concept that greatly simplifies the programming of collections through an API known as LINQ, for language integrated query. Finally, the chapter ended with the concept of expression trees, and how they compile into data that represents a lambda expression, rather than the delegate implementation itself. This is a key feature that enables such libraries as LINQ to SQL and LINQ to XML, libraries that interpret the expression tree and use it within contexts other than CIL.