The chemical kinetics of heterogeneous chemical reactions of importance in the atmosphere are discussed using six examples: 1) water vapor and 2) HCl interacting with ice, 3) ClONO2 + HCl on ice, 4) HONO + HCl on H2SO4 and ice, 5) HNO3 reacting on dry CaCO3 powder and 6) reactions of HNO3, N2O5; ClONO2, ClNO2 on salts such as NaCl and KBr. Quantitative results of uptake coefficients are presented and the importance of ionic displacement reactions at the interface is stressed.