This special issue brings together papers that describe some of the many ways that collaborative information seeking manifests itself. Some papers report on collaborative practices in a range of domains, including medical (Hertzum), legal (Attfield et al.), and online Q&A (Gazan). Others propose and evaluate models of collaborative activity (Evans and Chi; Evans et al.; Wilson and schraefel; Foley and Smeaton), and others describe systems and algorithms that support collaboration in various ways (Boydell and Smyth; Fernandez-Luna et al., Halvey et al., Morris et al.; Shah et al.).