In 1970 Merrill reviewed the application of molecular beams to studies of gas-surface interactions related to catalysis [1]. Since that time there has been a rather impressive increase in the number of experimental studies in this area. More recently, Saltsburg has reviewed the dynamical aspects of gas-surface collisions, with some emphasis on beam experiments [2]. The latest advances in experimental techniques in molecular beam studies of reactive scattering and heterogeneous catalysis have been reviewed by Smith [3] and most recently by Madix [4]. The purpose of this review is to present both a comprehensive survey of the experimental work that has been reported subsequent to the review by Merrill and to focus critical attention on what is felt to be the more significant features of this work. While our first intent can be carried out rather objectively, we have chosen to focus on those aspects of the work that relate more closely to our own research.