In response to rapid urbanization throughout the global South, urban and peri-urban slums are expanding at an alarming rate. Owing to inadequate financial and institutional resources at the municipal level, conventional approaches for safe water provision with centralized treatment and distribution infrastructure have been unable to keep pace with rapidly growing demand. In the absence of alternatives to centralized systems, a global public health emergency of infectious water-related diseases has developed. Alternative decentralized water treatment systems have been promoted in recent years as a means of achieving rapid health gains among vulnerable populations. Though much work with decentralized systems, especially in urban environments, has been at the household level, there is also considerable potential for development at the community level. Both levels of approach have unique sets of advantages and disadvantages that, just as with treatment technologies, may make certain options more appropriate than others in a particular setting. Integrating community, government and other relevant stakeholders into the process of systems development and implementation is essential if the outcome is to be appropriate to local circumstances and sustainable in the long term.