This chapter provides an overview of the synthesis, metabolism, transport, and signal transduction of abscisic acid (ABA), a key plant hormone. ABA, discovered over 50 years ago, regulates various aspects of plant growth and development, including seed maturation, germination, stress responses, and flowering. The chapter highlights the identification of nearly all enzymes involved in ABA metabolism and the discovery of over 200 loci regulating ABA response. ABA signaling mechanisms vary among tissues and developmental stages, and the chapter discusses these variations in the context of ABA's effects on seed maturation, germination, seedling growth, stress responses, and other processes. The chapter also covers the structure, biosynthesis, and metabolism of ABA, emphasizing the role of specific enzymes and regulatory steps. Additionally, it explores the identification of signaling intermediates, including protein phosphatases, transcriptional regulators, protein kinases, receptors, and diverse regulators with pleiotropic effects. The chapter concludes by discussing the functional roles of these components in ABA signaling and their interactions with other developmental and environmental signals.This chapter provides an overview of the synthesis, metabolism, transport, and signal transduction of abscisic acid (ABA), a key plant hormone. ABA, discovered over 50 years ago, regulates various aspects of plant growth and development, including seed maturation, germination, stress responses, and flowering. The chapter highlights the identification of nearly all enzymes involved in ABA metabolism and the discovery of over 200 loci regulating ABA response. ABA signaling mechanisms vary among tissues and developmental stages, and the chapter discusses these variations in the context of ABA's effects on seed maturation, germination, seedling growth, stress responses, and other processes. The chapter also covers the structure, biosynthesis, and metabolism of ABA, emphasizing the role of specific enzymes and regulatory steps. Additionally, it explores the identification of signaling intermediates, including protein phosphatases, transcriptional regulators, protein kinases, receptors, and diverse regulators with pleiotropic effects. The chapter concludes by discussing the functional roles of these components in ABA signaling and their interactions with other developmental and environmental signals.