This article traces the history of penetration testing from its military and intelligence roots in the 1960s to its formalisation through U.S. Tiger Teams and J.P. Anderson’s security frameworks. It follows the growth of pen testing into the commercial sector during the 1980s–90s, highlights key tooling milestones like SATAN, and explores its professionalisation in the 2000s via OWASP and PTaaS models. A dedicated UK section explains the roles of CESG, CHECK, CREST, and the NCSC in standardising and accrediting pen testing within British institutions. The article concludes with a reflection on how penetration testing continues to evolve in parallel with modern cyber threats.