Results of low energy electron diffraction studies of a variety of surface structures formed by chemisorption are reviewed. Ordered structures and "superstructures", reconstructive first-order transitions and order-disorder phenomena are found to be common. Isotherms with a continuous dependence of coverage on pressure do not apply to such "single crystal" systems but may apply accidentally to the polycrystalline systems. The Fowler and similar isotherms are more appropriate but neglect important elements of the complex interactions producing first-order transitions accompanied by reconstruction of substrate atoms. Factors determining surface structures are discussed. These structures are a prolific source of 1D and 2D order-disorder phenomena with wide range of complexity. Activated adsorption and nucleation phenomena are probably common but sometimes go unnoticed when less direct techniques are used because of initial unactivated adsorption in relatively unstable states, or because the more stable states are not attained. All these phenomena have assumed a considerable importance rather abruptly because experimental substantiation has required direct measurement of ordered surface structures.