In recent years researchers have found that people often collaborate during information seeking activities. Collaborative information seeking (CIS) is composed of multiple different activities like seeking, sharing, understanding, and using information together. However, most studies of CIS have focused on how people find and retrieve information collaboratively, while overlooking the important question of how people collaboratively understand the information found by different group members. This thesis focuses on a specific aspect of CIS, namely collaborative sensemaking, which is the question of how people together understand the information found during CIS activities. The term sensemaking has been used in a variety of disciplines and simply means "understanding the meaning of." Sensemaking is an important aspect of information seeking tasks. However, most of the sensemaking research has been at the individual level and there is little understanding of how sensemaking takes place in collaborative work, specifically collaborative information seeking. In this thesis, I address two important gaps in current research on sensemaking in CIS activities. First, there is a lack of conceptual understanding about why and how people collaborate to understand the information found during CIS activities. Second, developers of collaborative information retrieval tools have rarely focused on helping users of such tools make sense of the information found. To address these research gaps I undertook a multi-method research approach in which I conducted two studies in two different CIS domains. The first was an ethnographic study of the CIS activities of healthcare providers working in the emergency department of a large teaching hospital. In this study, I used qualitative methods like interviews, observations, shadowing, and artifacts collection to examine how groups collaboratively find, understand, and use information in a highly collaborative and information-intensive environment. The second study was conducted in the domain of collaborative Web search where I examined the search and sensemaking behavior of users of collaborative Web search tools. Through lab studies and the development of a tool, CoSense, I examined how collaborative sensemaking can be supported during Web search tasks. Through these studies, I provide two important contributions to our understanding of sensemaking in CIS activities. First, I expand our conceptual understanding of collaborative sensemaking by highlighting the occasions and characteristics of collaborative sensemaking in CIS activities and by presenting a framework of collaborative sensemaking. Second, I provide insight into the design-features that can support sensemaking in collaborative information retrieval tools and also the challenges in designing such features. The research presented in this thesis helps us extend our conceptual understanding of collaborative sensemaking and also provides insight into how collaborative sensemaking can be supported in collaborative information retrieval tools. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]