The identification of suitable sites for marine aquaculture is an extremely important element in a successful commercial operation. The characterization of sites, in terms of exposure levels for systems and stock, will directly determine the options for use, and the types of technology and operating system which may be considered. For offshore mariculture, potentially with higher levels of routine exposure, such criteria may become critical. Based on longstanding experience at surveying and developing aquaculture sites in N Europe and the Mediterranean, this paper outlines some of the practical issues to be considered in evaluating sites and specifying their operational characteristics. Concepts are also developed to define the suitability of various cage system types for different site environments. Information is also provided on retrospective data to be obtained from loss analysis.