This book reviews current understanding of the biological roles of the signal transducer and activator of transcription (STAT) proteins and their dysregulation in diseases. STAT proteins were named after their role as signal transducers and activators of transcription. STAT proteins are highly conserved among species, thus reflecting the importance of these transcription factors in controlling processes such as development, cell growth and homeostasis. STAT1 and STAT2 were the first STAT genes identified in the interferon signal transduction pathways. Since then seven more STATs have been identified including STAT3, STAT4, STAT5A, STAT5B and STAT6. The enormous amount of studies on STAT proteins over the past decade has enhanced our understanding of how STAT proteins at the biochemical and molecular level regulate gene expression.