Breast cancer is the most common cause of cancer death in women worldwide. This malignancy is a complex disease, which is defined by an intrinsic heterogeneity on the histopathological and molecular level as well as response to therapy and outcome. In addition to classical histopathological features, breast cancer can be categorized into at least five major subtypes based on comprehensive gene expression profiling: luminal A, luminal B, basal-like, ERBB2-positive, and normal-like breast cancer. Genetically engineered mouse models can serve as tools to study the molecular underpinnings for this disease. Given the genetic complexity that drives the initiation and progression of individual breast cancer subtypes, it is evident that certain models can reflect only particular aspects of this malignancy. In this book chapter, we will primarily focus on advances in modeling breast cancer at defined stages of carcinogenesis using genetically engineered mice. We will discuss the ability as well as shortcomings of these models to faithfully recapitulate the spectrum of human breast cancer subtypes.