Password generators that generate site-specific passwords on demand are an alternative to password managers. Over the last 15 years a range of such systems have been described. We propose the first general model for such systems, and critically examine options for instantiating it. The model enables an objective assessment of the design of such systems; it has also been used to sketch a possible new scheme, AutoPass, intended to incorporate the best features of the prior art while addressing many of the shortcomings of existing systems.