Volunteers and charitable organizations contribute significantly to community welfare through their prosocial behavior: that is, discretionary behavior such as assisting, comforting, sharing, and cooperating intended to help worthy beneficiaries. This essay focuses on prosocial behavior on the Internet. It describes how offline charitable organizations are using the Net to become more efficient and effective. It also considers entirely new models of Net-based volunteer behavior directed at creating socially beneficial information goods and services. After exploring the scope and diversity of online prosocial behavior, the essay focuses on ways to encourage this kind of behavior through appropriate task and social structures, motivational signals, and trust indicators. It concludes by asking how local offline communities ultimately could be diminished or strengthened as prosocial behavior increases online.