NBitcoin is a comprehensive .NET library for Bitcoin and altcoin protocol support. It provides low-level primitives for transactions, blocks, keys, scripts, addresses, HD wallets (BIP32/39), PSBT (BIP174/370), Taproot/Schnorr, and a full P2P networking stack. The library also includes internal cryptographic implementations (secp256k1, SHA-256, BouncyCastle) and supports 30+ altcoins via custom network definitions and consensus factories. It is used in production by BTCPay Server, Wasabi Wallet, and other Bitcoin infrastructure. The architecture is a set of interconnected NuGet packages (NBitcoin, NBitcoin.Altcoins, NBitcoin.Secp256k1, NBitcoin.TestFramework, NBitcoin.Bench) with extensive integration tests that run against real Bitcoin Core nodes. While the core library is well-engineered with clean abstractions, the altcoin surface introduces some duplication and conditional compilation complexity, and the internal BouncyCastle fork may diverge from upstream. Overall, it is a high-quality, actively maintained library with strong reusability of its core components.