Abstract: It is widely assumed that all irrigation systems must have constituted authority and that all large irrigation systems must have centralized authority. The small literature which tests these beliefs is reviewed, and the results of a systematic comparative study are presented. The concepts of “irrigation system,” “irrigation system size,” and “irrigation system structure of authority” are defined. Variables which measure these concepts are constructed, and measurements from a purposive sample are displayed. A handful of small irrigation systems without authority structures was found. An inspection of the data reveals no relationship between size and the structure of authority in systems ranging from 700 to 458,000 ha. Furthermore, an irrigation system of 458,000 ha is managed by farmers. Conclusions: (1) irrigation systems without constituted authority exist and (2) large systems do not require central authority.