The articles contained in this special issue of Human- Computer Interaction were originally presented at a workshop entitled Human-Computer Interaction in the 21st Century: Prospects and Visions. The workshop marked both the 10th anniversary of the Human-Computer Interaction Consortium and the end of the millennium. The papers reflect the field's evolution toward more complex and contextualized views of interactive systems and their use. This consideration of more realistic tasks, models, and situations of use can be seen in the various domains of inquiry represented here, from task-action models in the GOMS family to support for collaborative work. The papers collectively offer a set of proposals for the future: new agendas that can take research in human-computer interaction and computer-supported cooperative work to the next level.