Book excerpt: Direct Modeling for Computational Fluid Dynamics -- Introduction to Gas Kinetic Theory -- Introduction to Nonequilibrium Flow Simulations -- Gas Kinetic Scheme -- Unified Gas Kinetic Scheme -- Low Speed Microflow Studies -- High Speed Flow Studies -- Unified Gas Kinetic Scheme for Diatomic Gas -- Conclusion -- Appendix A: Non-dimensionalizing fluid dynamic variables -- Appendix B. Connection between BGK, Navier Stokes and Euler equations -- Appendix C. Moments of Maxwellian distribution function and expansion coefficients -- Appendix D. Flux evaluation through stationary and moving cell interfaces