Much of our current state of knowledge pertaining to the mechanisms controlling intestinal epithelial homeostasis derives from epidemiological, molecular genetic, cell biological, and biochemical studies of signaling pathways that are dysregulated during the process of colorectal tumorigenesis. Activating mutations in members of the RAS oncoprotein family play an important role in the progression of colorectal cancer (CRC) and, by extension, intestinal epithelial homeostasis. Mutations in K-RAS account for 90% of the RAS mutations found in CRC. As such, the study of RAS protein function in the intestinal epithelium is largely encompassed by the study of K-RAS function in CRC. In this review, we summarize the data available from genetically defined in vitro and in vivo models of CRC that aim to characterize the oncogenic properties of mutationally activated K-RAS. These studies paint a complex picture of a multi-functional oncoprotein that engages an array of downstream signaling pathways to influence cellular behaviors that are both pro- and anti-tumorigenic. While the complexity of K-RAS biology has thus far prevented a comprehensive understanding of its oncogenic properties, the work to date lays a foundation for the development of new therapeutic strategies to treat K-RAS mutant CRC.