Abstract: Rarefied gas dynamics serves as an essential bridge connecting molecular-scale motion to macroscopic fluid transport behavior, with broad applications in spacecraft re-entry flight, hypersonic aerodynamics, and micro/nano-scale transport. When the molecular mean free path becomes comparable to the characteristic flow length, the continuum assumption fails, and significant non-equilibrium transport effects emerge. At gas-solid interfaces, velocity slip and temperature jump occur, where the equilibrium distribution condition no longer holds, and the classical Navier–Stokes equations fail to accurately describe the flow. The Boltzmann equation, as the fundamental governing equation for non-equilibrium rarefied gas flow, describes the statistical laws of molecular transport and, under certain conditions, is capable of capturing flow phenomena across the entire regime from free molecular to continuum flow. To enable efficient solutions of the Boltzmann equation, researchers have proposed various approximate modeling and simplification strategies, including macroscopic closure models, particle-based methods and direct simulation Monte Carlo, discrete velocity methods and mesoscopic gas-kinetic schemes, as well as hybrid algorithms developed in recent years. Another critical issue in rarefied gas flows concerns the modeling of gas–solid interactions and the associated boundary conditions. Accurately characterizing these interactions across micro-, meso-, and macroscopic scales is essential for the predictive simulation of multi-scale flows spanning different regimes. This paper reviews and synthesizes recent progress in both theoretical approaches to rarefied gas dynamics and the modeling of gas–solid boundary conditions. It is further identified that the core challenge in current research lies in effectively establishing simplified Boltzmann formulations and their corresponding boundary conditions that strike a balance between predictive fidelity and computational cost. Addressing this challenge is of substantial theoretical and practical value for advancing rarefied gas dynamics and supporting aerospace engineering applications, including very-low-Earth-orbit satellites and deep-space exploration.