Designers of building services systems are frequently called upon to select the most appropriate air-conditioning system for a specific application. To carry out the selection process correctly, they must have a good knowledge of the various air-conditioning systems available on the market, their characteristics and applications. They must also be aware of the implications of selecting a particular system on the capital and running costs. Other important criteria in the selection process include spatial requirements, maintainability, reliability, flexibility and environmental impacts. All these factors are interrelated and so the designer should understand their relative importance for each design job. Other important selection criteria include the specific requirements of the client in terms of aesthetics, maximisation of rental income and the saleability of the property.