Segmentation or abnormality detection is the initial step in mammographic computer-aided diagnosis (CAD) systems. In this chapter we review different approaches to the segmentation of mammographic masses, describing their main features and highlighting the differences among them. The key objective is to point out the advantages and disadvantages of these approaches. In contrast with other reviews, which only describe and compare different approaches qualitatively, in this chapter we also provide a quantitative comparison. Using a public database, the performance of eight relevant methods are compared using Receiver Operating Characteristic (ROC) and Free-Response Receiver Operating Characteristic (FROC) analysis.