Introduction This chapter describes how to identify source lines of code (SLOC). It addresses the many different facets of lines and gives clear direction on the proper use and deﬁnition of lines. Included in these discussions are physical and logical source lines and a checklist for source line deﬁnitions. The chapter also describes how to handle issues and risks associated with using SLOC to determine program size. It explains how the SLOC sizing approach is affected by different computer languages, describes best practices for approaching size for different languages, and explains the proper use of the size metric. Throughout this chapter information, will be provided for counting SLOC for a completed program and for a future program in which SLOC must be estimated.