Monorepos, which consolidate the code of multiple interrelated projects into a single repository, are increasingly popular in the software development landscape for their organizational benefits, but they also introduce tooling challenges. This comprehensive examination of monorepo build tools compares four prominent options: Bazel, Pants, Nx, and Earthly, each with distinct features regarding programming language support, learning curve, caching, remote execution, build introspection, and versatility. Bazel is renowned for its speed and scalability, making it suitable for large codebases, though it has a steep learning curve. Pants offers a more Python-friendly alternative with simplified adoption processes, while Nx focuses on JavaScript and TypeScript projects, providing a user-friendly experience for frontend developers. Earthly, leveraging a containerized approach akin to Docker, offers flexibility for multi-language monorepos and is ideal for complex builds requiring system-level dependencies. Each tool has strengths for specific use cases, highlighting the importance of evaluating organizational needs and existing workflows when choosing a build tool.