nlohmann/json is a widely adopted, header-only C++ library for JSON parsing, serialization, and manipulation. It provides a single-header `json.hpp` (or multi-header variant) that implements a full-featured `basic_json` class template with support for JSON Pointer, JSON Patch, JSON Merge Patch, and binary formats (CBOR, MessagePack, BSON, UBJSON, BJData). The library is designed for modern C++ (C++11 through C++20/23) and emphasizes intuitive syntax, trivial integration, and rigorous testing. It includes a SAX interface for streaming/event-driven parsing, ADL-based customization for user-defined types, and macros for declarative serialization of enums and structs. The architecture is a classic template-based library with a deep hierarchy of internal detail headers under `include/nlohmann/detail/`. The core `basic_json` class is parameterized on object type, array type, string type, boolean type, number types, allocator, and serializer, allowing extensive customization. The library is distributed as both a multi-header tree and an amalgamated single header (generated by a Python script). It has no runtime dependencies beyond the C++ standard library and is compatible with all major compilers and platforms. Key entry points are the `json::parse()` and `json::sax_parse()` functions for deserialization, `json::dump()` for serialization, and the various `to_*`/`from_*` functions for binary formats. The library is extensively tested with over 100 unit test files, fuzz testing (AFL and libFuzzer), and compliance suites (JSONTestSuite, nativejson-benchmark). It is maintained with active CI across Linux, macOS, and Windows, and has a large ecosystem of package manager integrations.