This paper is a concise, English-language overview of Spec-Driven Development (SDD), a software development methodology that treats the specification as the primary artifact—the living documentation itself—from which code and tests are derived, with humans determining intent and AI and tools performing generation. SDD is systematized in the Japanese book "Spec-Driven Development: A Practical Introduction" (Nikkei BP, 2026); because that full treatment is currently available only in Japanese, this primer is intended to give English-speaking readers the foundation needed to follow it—and, in particular, to serve as the foundation for a companion paper that proposes Spec-Driven Architecture. This paper summarizes SDD through its Three Technical Elements, its Four Principles, and its Seven Processes, and points readers to the book for the complete account.