In 1994, Kuhajda and colleagues unambiguously identified the oncogenic antigen-519, a prognostic molecule found in breast cancer patients with markedly worsened prognosis, as fatty acid synthase (FAS), the key enzyme for the de novo fatty acid biosynthesis. It now appears that human carcinomas and their pre-neoplastic lesions constitutively overexpress FAS and undergo significant endogenous fatty acid biosynthesis. Moreover, FAS blockade specifically induces apoptotic cancer cell death and prolongs survival of cancer xenograft hosts. Therefore, FAS signaling seems to play a central role in the maintenance of the malignant phenotype by enhancing cancer cell survival and proliferation. This review documents the rapidly changing perspectives on the function of FAS in cancer biology. First, we describe molecular mechanism by which aberrant transduction cascades driven by oncogenic changes subvert the down-regulatory effects of dietary fatty acids, resulting in tumor-associated FAS insensitivity to nutritional signals. Second, we speculate on the putative function that hypoxia can play as the epigenetic factor that triggers and maintains FAS overexpression in cancer cells by inducing changes in gene expression and in metabolism for survival. Third, we explore the role that FAS exhibits in cancer evolution by specifically regulating cancer-related proteins such as Her-2/neu oncogene and estrogen receptor. Finally, we reveal previously unrecognized functions of FAS on the response of cancer cells to chemo-, endocrine-, and immuno-therapies. These findings, all together, should ultimately enhance our understanding of how FAS-dependent endogenous fatty acid metabolism, once considered a minor anabolic-energy-storage pathway in normal cells, has become a jack-of-all-trades in cancer cells.