This chapter describes how social politeness is relevant to computer system design. As the Internet becomes more social, computers now mediate social interactions, act as social agents, and serve as information assistants. To succeed in these roles computers must learn a new skill—politeness. Yet selfish software is currently a widespread problem and politeness remains a software design “blind spot.” Using an informational definition of politeness, as the giving of social choice, suggests four aspects: 1. respect, 2. openness, 3. helpfulness, and 4. remembering. Examples are given to suggest how polite computing could make human-computer interactions more pleasant and increase software usage. In contrast, if software rudeness makes the Internet an unpleasant place to be, usage may minimize. For the Internet to recognize its social potential, software must be not only useful and usable, but also polite.