A loop is a basic logical programming concept where one or more statements are executed repetitively until a predefined condition is satisfied. Loops are used in all programming languages. Compared to other programming languages, loop processing is more complex in SAS® because of its unique implicit loop. The implicit loop is not an easy construct to grasp, even by programmers with advanced skills in other programming languages. Knowing when the time is right to create an explicit loop can also present a challenge to the SAS programmer who has to work simultaneously with the implied loop in the DATA step. The material in this chapter is based on a paper that I presented at SAS Global Forum (Li, 2010).