WebAssembly (Wasm) is a safe and portable code format used in a broad variety of computational environments, such as Web browsers, cloud, edge, IoT, embedded systems, and blockchains. As a low-level programming language its instruction set is close to that of physical hardware, yet its semantics enforces memory safety, isolation, and the absence of undefined behavior. A distinguishing feature of Wasm is that its official specification contains a complete formal semantics, based directly on techniques developed and established by the scientific community, and proved sound with machine-verified proofs. Its popularity as a technology for both practically building and theoretically investigating verified and secure systems has hence been growing rapidly. With this Dagstuhl Seminar, we intend to bring together all sides interested in Wasm, its formal semantics, and its application to verification and generation techniques.