The JAK/signal transducers and activators of transcription (STAT) pathway serves as a pivotal signaling axis governing a plethora of cellular activities, modulated by an array of ligands such as hormones, cytokines, and growth factors. It orchestrates vital processes including cell survival, apoptosis, and hematopoiesis, while also exerting profound influence on immune regulation. Dysregulation of this pathway is implicated in a spectrum of disorders spanning immunological, hematologic, and oncological domains. Oncogenic mutations targeting receptors, downstream effectors, and transcriptional factors contribute to the aberrant activation of JAK/STAT signaling, fostering the development and progression of various cancers. Within malignancy, STAT proteins exhibit multifaceted roles, with certain members implicated in diverse tumor types, while others play pivotal roles in tumor initiation and progression. Notably, STAT3 and STAT5 emerge as central players in these processes. Given its pivotal role, intensive research into the JAK/STAT pathway is warranted, particularly for its potential as a target for cancer therapies and drug development. This chapter illuminates the composition, functional roles, and activation dynamics of the JAK/STAT pathway in different cancer types, shedding light on therapeutic implications and strategies aimed at modulating tumor progression by targeting STAT proteins.