In a growing number of countries, guidelines are playing an increasingly important role in assuring the quality of care. Their validity depends on a systematic development process and explicit links between recommendations and underlying evidence. Their role in aiding clinical decision making depends on their developers identifying the key decisions and their consequences; gathering the relevant evidence on the risks and costs of alternative decisions; and presenting the appropriate evidence to make each key decision in a simple and accessible format, possibly electronic. Decision analysis is a potentially powerful tool for clarifying clinical decisions and involving patients directly in the process but its routine use in guidelines is complex and has yet to be fully evaluated. Duplication of guidelines can be avoided by appraising and adapting existing guidelines in local contexts. There is very little evidence available about the impact of guidelines on the doctor-patient relationship. They might have a potentially deleterious effect, but the combination of explicit guidelines eliciting patient preferences and information technology might redress the balance by increasing the role of patients themselves.