This chapter provides a review of the basic ideas at the core of the Computational Singular Perturbation (CSP) method and the Tangential Stretching Rate (TSR) analysis. It includes a coherent summary of the theoretical foundations of these two methodologies while emphasizing theirmutual interconnections. The main theoretical findings are presented in a systematic fashion. Their virtues and limitations will be discussed with reference to auto-ignition systems, laminar and turbulent premixed flames, and non-premixed jet flames. The material presented in the chapter constitutes an effective guideline for further studies.