WebAssembly (abbreviated as Wasm) was initially introduced for the Web and quickly extended its reach into various domains beyond the Web. To create Wasm applications, developers can compile high-level programming languages into Wasm binaries or manually write the textual format of Wasm and translate it into Wasm binaries by the toolchain. Regardless of whether it is utilized within or outside the Web, the execution of Wasm binaries is supported by the Wasm runtime. Such a runtime provides a secure, memory-efficient, and sandboxed execution environment to execute Wasm binaries. This article provides a comprehensive survey of research on Wasm runtimes with 103 collected research papers related to Wasm runtimes following the traditional systematic literature review process. It characterizes existing studies from two different angles, including the internal research of Wasm runtimes (Wasm runtime design, testing, and analysis) and the external research (applying Wasm runtimes to various domains). This article also proposes future research directions about Wasm runtimes.