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.

Visualization and User Control of Recommender Systems.- Crowdsourcing and Knowledge Co-creation in Virtual Museums.- Using real-time gaze sharing methods to enhance collaboration.- Evaluating Anchored Discussion to Foster Creativity in Online Collaboration.-Measuring the effort demanded by CSCL design processes supporting a consistent artifact flow.-Supporting Collaborative Decision Making in Geo-collaboration Scenarios.- Analysis of Question and Answering Behavior in Question Routing Services.- An Integrative Tool Chain for Collaborative Virtual Museums in Immersive Virtual Environments.- A Behaviour Awareness Mechanism to Support Collaborative Learning.- Exploiting the use of wikis to support collaborative writing: a case study of an undergraduate Computer Science class.- Where to begin? Using Network Analytics for the Recommendation of Scientific Papers.- Every answer has a question: exploring communication and knowledge exchange in MOOCs through learning analytics.- Dynamic Credibility Threshold Assignment in Trust and Reputation Mechanisms Using PID Controller.- XCuteKIP: Support for Knowledge Intensive Process Activities.- BESIDE: Immersive System to Enhance Learning within a Museum.- Secure Collaboration in Public Cloud Storages.- Sketchpad: A learning tool supporting creativity in collaborative learning activities.- A Subscription Overlay Network for Large-Scale and Efficient File.-Parallel Downloading.- Synchronizing Dining Progress in Video-Mediated Time-shifted Table Talk Induces More Engagement.- Contrasting People's Attitudes towards Self-disclosure in Online Social Networks and Face-to-face Settings.

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.