The use of functions are something that, we are taught in our first programming lesson, are good development practice, to encapsulate common code and ensure code reuse. In this session we will look at the different ways you can write functions in SQL Server and more importantly look at the performance and pitfalls of each option. Unfortunately the use of functions in SQL Server can seriously damage your performance. In this session we will look at why functions are used and how you can get around the limitations of the current implementation of functions and still maintain performance. This session is essential for anyone using functions in batch type scenarios such as reports, ETL and batch processing. After this session you will see how you can increase your query performance 100 fold by employing some simple practices.