This book outlines a methodology to develop formally verified Just-in-Time compilers. Just-in-Time compilation is a technique to execute programs, where execution is interleaved with optimizations of the program itself. These compilers often produce fast executions, so much so that their use has grown greatly for dynamic programming languages. Most modern web browsers today use Just-in-Time compilation to speed up the execution of the JavaScript programs they execute. However, the techniques used in Just-in-Time compilers can be particularly complex. This complexity can be a source of bugs and vulnerabilities. How can you make sure that your Just-in-Time compiler is bug-free? For traditional ahead-of-time compilers, many techniques have been developed to prevent compilation bugs. One such technique is formally verified compilation, where the compiler itself comes with proof that the semantics of the compiled program correspond to the semantics of the source program. But Just-in-Time compilers are more recent, less understood, and have been the target of far fewer verification efforts. To bring formal verification to Just-in-Time compilation, the book identifies a set of specific verification challenges and presents novel solutions for each of them. Such challenges include dynamic optimizations, speculative optimizations, deoptimizations, and the interleaving of interpretation and machine code generation. The author repurposes proof techniques from formally verified ahead-of-time compilers like CompCert. Following this methodology, readers can develop Just-in-Time compilers and formally prove that they behave as prescribed by the semantics of the program they execute. All proofs within the book have been mechanized in the Coq proof assistant.