Security is often seen at the point of contact: a uniform at a door, a camera above an entrance, a key, a barrier or a control room. But the decisions that make protection succeed or fail usually begin much earlier. From Lock and Key to Martyn's Law traces the development of security in Britain from custody, watch and ward and the growth of public policing through industrial protection, contract security, public events, licensing, surveillance, data and modern organisational responsibility. It follows the changing relationship between people, technology, authority and management, showing how security grew not by replacing one layer with another, but by adding new responsibilities to those that already existed. The book's central contemporary section examines the Manchester Arena attack and Inquiry, Figen Murray's campaign for change, and the development of the Terrorism (Protection of Premises) Act 2025—Martyn's Law. It then considers what modern public protection means for security officers, private security companies, installation providers, venue managers, event organisers and university security departments. The final part looks beyond compliance to rights, public trust, artificial intelligence, biometrics, automation, digital identity and the next generation of security systems. Written as evidence-led professional nonfiction rather than a compliance manual, this book is for security practitioners, managers, responsible persons, students and readers interested in how security became a modern organisational and public-protection function.