It is not so difficult to detect adsorbed atoms and molecules on solid surfaces, but their reaction processes are still difficult to direct. Catalysis is caused by the ensemble operation of their reaction via intermediates on active sites, and suitable elementary reactions are supposed to explain the catalysis. Accordingly, unsettled problems in catalysis are caused by our lack of knowledge about the dynamics of elementary processes. In this paper, activity of the catalyst is discussed from the following points; (i) activation by restructuring, (ii) activation by forming active sites and reactive intermediates, (iii) site dependent turnover frequency, and (iv) role of promoting materials. (C) 2010 Elsevier By. All rights reserved.