This paper reviews two physical chemistry topics related to atmospheric chemistry. First, we describe semiempirical models, based on thermochemistry and transition state theory, which are employed to estimate gas phase reaction rates over a wide range of temperatures and pressures. We also review briefly the experimental techniques utilized for measurements of elementary reaction rate constants, which provide the primary input to these models. We then address chemical reactions which lake place in the atmosphere on the surface of solid, ice-like aerosol particles, discussing some current views on the mechanisms of these reactions and describing some laboratory techniques for the study of these heterogeneous processes.