The T cell-specific DNA-binding protein TCF-1 is a central regulator of T cell development and function along multiple stages and lineages. Because it interacts with β-catenin, TCF-1 has been classically viewed as a downstream effector of canonical Wnt signaling, although there is strong evidence for β-catenin-independent TCF-1 functions. TCF-1 co-binds accessible regulatory regions containing or lacking its conserved motif and cooperates with other nuclear factors to establish context-dependent epigenetic and transcription programs that are essential for T cell development and for regulating immune responses to infection, autoimmunity and cancer. Although it has mostly been associated with positive regulation of chromatin accessibility and gene expression, TCF-1 has the potential to reduce chromatin accessibility and thereby suppress gene expression. In addition, the binding of TCF-1 bends the DNA and affects the chromatin conformation genome wide. This Review discusses the current understanding of the multiple roles of TCF-1 in T cell development and function and their mechanistic underpinnings.