Requires.jl is a Julia library that provides a mechanism for conditional code loading based on the presence of other packages. It exports the `@require` macro, which allows package authors to define glue code that only executes when a specific dependency is loaded, avoiding explicit hard dependencies and reducing load times. The library hooks into Julia's internal package loading system via `Base.package_callbacks` and uses a callback pattern to defer execution. It also includes an `@init` macro for deferred initialization and an `@include` macro that caches file content at compile time for use in system images. The package is widely used in the Julia ecosystem to manage optional dependencies and is considered a precursor to Julia's native package extensions (introduced in Julia 1.9). The code is clean, well-structured, and demonstrates non-obvious skill in manipulating Julia's module loading internals.