Publisher Summary Particle transport refers to the diffusion or transport of small particles, such as protons, neutrons, photons, electrons, ions, or even neutral atoms, within some host medium. This chapter discusses how the strength of a radiation field of neutral particles, such as neutrons and photons, is characterized by the flux density; how the flow across a surface is characterized by the current vector; how the interaction coefficient determines the probability a radiation particle interacts with the host medium; and how the reaction rate per unit volume of the radiation with the medium is given. It is shown that uncollided neutral-particle radiation is attenuated exponentially as it traverses the host medium. The interaction coefficient and associated microscopic cross sections for both photons and neutrons are illustrated. Two forms of the linearized Boltzmann equation are derived that describe exactly how radiation migrates through a host medium. The integrodifferential form, which is widely used in deterministic transport calculations, was then converted into an integral form, the form first used in Monte Carlo analysis of radiation transport. Both the direct and adjoint integral forms are introduced in the chapter. The adjoint solution plays an important role in maximizing variance reduction of Monte Carlo evaluation of the direct solution.