fontspec is a mature LaTeX package that provides an automatic and unified interface for font selection in XeLaTeX and LuaLaTeX. It allows users to load OpenType, AAT, and Graphite fonts by name or filename, with full control over features such as ligatures, small caps, numerals, script/language selection, colour, scaling, and more. The architecture is modular: a core LaTeX3 (expl3) implementation wraps engine-specific code (XeTeX primitives or luaotfload) behind a consistent key-value interface, and provides both user commands (`\setmainfont`, `\fontspec`) and a programmable API (`\fontspec_*` conditionals) for other packages. The codebase is structured as literate `.dtx` files, generating both the package `.sty` files and comprehensive documentation. It includes an extensive test suite with over 100 test files that run on both XeTeX and LuaTeX, a fully automated CI/CD pipeline (testing, documentation, release), and is distributed on CTAN under the LPPL 1.3c license. The package is actively maintained by the LaTeX3 Project team, tracking OpenType specification updates and TeX engine changes, though its dual-engine support and large static data tables (language/script mappings) introduce some maintenance overhead.