Since the late nineteenth century, people have given more consideration to public toilet systems (also known as restrooms, bathrooms, washrooms, or sanitary systems). These kinds of systems and facilities are important to people's health, while also an inseparable part of our everyday life. Generating creative (and sui- table) designs for public toilet systems and facilities is always the objective and dream of environmental and product designers. One of the major reasons is that "public" is a very challenging topic, as users are not identical. Another major re- ason is that toilet systems are an area related to changing behavior, which is con- tinuously affected by different and also changing physiological, psychological, social, cultural and ideological factors. According to the case studies on public toilets in Hong Kong and other cities, this paper initiates the 6-Ps Principle: Par- ticipation, Policy, Plan, Preparation, Promotion, Practice. To deal with conti- nuously changing human expectations, preferences and behavior, the Principle can be considered an "action approach" to the design (or thinking and consideration) process for the implementation of public toilet systems and facilities. The paper identifies the importance of each element (or stage) of the Principle, and discusses how these elements are linked to each other.