Performance monitoring and optimization is an inseparable part of a DBA's activities. To optimize a poorly performing system/query or to troubleshoot the performance issues you need to know the root cause. Prior to SQL Server 2005, we had several system tables to monitor the state of the system.
Greg Larsen introduces some Dynamic Management Views (DMVs) and Dynamic Management Functions (DMFs) to help monitor your application's T-SQL performance. In addition, he provides a Stored Procedure that utilizes the DMVs and DMFs to produce a report that quickly identifies poorly performing T-SQL statements.