Krüppel-like factor 4 is a zinc finger-type transcription factor expressed in a variety of tissues, including the epithelium of intestine and the skin, where it is important in differentiation and cell cycle arrest. KLF4 can both activate and repress transcription, depending on the gene targeted. Moreover, KLF4 can function as a tumor suppressor or an oncogene, depending on the cellular context. Finally, KLF4 is important in reprogramming differentiated fibroblasts into inducible pluripotent stem cells, which highly resemble embryonic stem cells. This review will summarize what is known about the diverse functions of KLF4, as well as their molecular mechanisms.