This book constitutes the refereed proceedings of the 21st International Conference on Collaboration and Technology, CRIWG 2015, held in Yerevan, Armenia, in September 2015.

The 19 revised papers presented together with 1 invited talk were carefully reviewed and selected from 28 submissions. CRIWG has been focused on collaboration technology design, development, and evaluation. The background research is influenced by a number of disciplines, such as computer science, management science, informationsystems, engineering, psychology, cognitive sciences, and social sciences.