This paper examines technologies as rule-following behaviour, arguing that emerging practices define ‘rules’, rather than rules controlling practices. With the aid of several examples, it suggests that technologies should be conceptualized as extensive, open-ended technical-social systems whose local behaviour is underdetermined by any overall rationality. Contextual normalization of working technologies takes place according to local rationalities, but this may fragment the overall technology, whilst evolving its informal practical “rules’. Expert and public discourses present a more rule-bound concept of technology than the more private, contingent world of practice. The implications for public decisions and social control of technology are examined.