ForwardDiff.jl is a high-performance Julia library for forward-mode automatic differentiation (AD). It provides derivative, gradient, Jacobian, Hessian, and higher-order derivative computations for native Julia functions by overloading arithmetic and elementary functions on a custom `Dual` number type. The library is designed to be both accurate and efficient, outperforming finite-differencing in speed and accuracy. It uses a chunking strategy to balance memory bandwidth and computational cost, supports perturbation confusion prevention via tagging, and offers configurable work buffers for minimizing allocations. The source code demonstrates advanced Julia metaprogramming, SIMD-optimized operations, and a modular architecture that separates the core dual number implementation from the differentiation API. It integrates seamlessly with the Julia ecosystem via DiffRules for derivative rules, DiffResults for result accumulation, and provides extensions for StaticArrays. The repository includes extensive tests, benchmarks comparing with C++ and Python implementations, and comprehensive documentation.