This is the first in a series of papers devoted to the ecology of the sandy beaches of the Cape Peninsula. Conditions on these beaches are described, with special reference to the beaches of Hout Bay, Llandudno, Milnerton and Muizenberg. Tidal conditions, wave-action, beach profiles and sand movements are discussed, while attention is also given to the particle-size distribution of the substrata forming the beaches. The organic content of the sand, its permeability and porosity, and the disposition of the water-tables are all discussed briefly and some data concerning salinities, temperature and pH are presented. Mention is made of the fauna and a list of the more important members of the resident fauna of the beaches accorded special attention is given. There is some attempt to show how the physical factors are interrelated.