In this chapter various sanitation systems are introduced with a brief indication of their suitability for particular situations, the constraints on their use, and their disadvantages. The whole range of options is covered, including off-site systems and some that are not recommended because of the associated health risk and other disadvantages. Each community must choose the most feasible and convenient option to provide necessary health protection. Selecting the most appropriate option requires a thorough analysis of all factors including cost, cultural acceptability, simplicity of design and construction, operation and maintenance, and local availability of materials and skills. Further details of the design, construction, operation and maintenance of these systems are given in Part II.