Transport theory is a field of physics and applied mathematics dedicated to the study of the movement and distribution of particles, energy, or other conserved quantities within a medium. It employs kinetic equations, such as variations of the Boltzmann equation, to model the phase-space density and interactions (like scattering, absorption, and emission) of these entities, providing fundamental insights necessary for analyzing and predicting behavior in diverse systems, including nuclear reactors, astrophysical plasmas, and atmospheric physics.