The mathematics underlying zero-knowledge proofs has been understood since the 1980s. The computational cost of generating and verifying them was, for most of that period, prohibitive for practical applications at scale. What changed over the past several years was not the theory but the engineering: proof generation times dropped by orders of magnitude, hardware acceleration made ZK computation economically viable, and a generation of cryptographers trained in both theory and systems engineering turned their attention to making the technology work in production.