The present book deals with more than thirty years of research performed in Chemical Physics with uniform supersonic flows and CRESU-like apparatuses. As such, it is linked to processes occurring at temperature close to absolute zero, studied for their theoretical interest or for their key role in ultracold media. The goal of this first chapter is to introduce a few concepts and equations that are ubiquitous within this context. The leading thread is the question of Local Thermodynamic Equilibrium (LTE) in dilute gases, complete or partial, and how it is implied in kinetic processes (Section 1.2), cold astrophysical media (Section 1.3), laboratory methods (Section 1.4), and the derivation of hydrodynamic equations (Section 1.5). It has been chosen to use the Boltzmann equations to derive the link between microscopic quantities (as state-to-state cross sections) and macroscopic quantities (as rate coefficients). How they are used in the derivation of hydrodynamic equations is also highlighted. The key role of distribution functions for translational or internal energies either in astrophysical media (Section 1.3) or laboratory methods (Section 1.4) is discussed. Note that the concept of temperature is itself subtle and is discussed at length in Section 1.2. © 2022 by World Scientific Publishing Europe Ltd.