Ceres Solver is a production-grade C++ library for solving large-scale non-linear least squares problems, widely used in computer vision (bundle adjustment, SLAM) and robotics. It provides a flexible API where users define cost functions (analytical, automatic, or numerical differentiation), residual blocks, and parameter manifolds (Euclidean, quaternion, sphere, etc.), then solve via trust-region or line-search methods backed by a variety of sparse/dense linear solvers (Eigen, SuiteSparse, CUDA, Apple Accelerate). The architecture is modular: a core minimizer orchestrates strategy objects (Levenberg-Marquardt, Dogleg, BFGS), while evaluators and Jacobian writers support block-sparse and dynamic sparsity. The library also includes utilities like thread pools, concurrent queues, and graph algorithms for visibility-based preconditioning. Despite a complex build matrix with optional backends, the code is well-structured, heavily tested, and actively maintained.