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.