The four studies presented show how expertise, from the cultural-historical theory of activity, is constructed interactively in everyday problem situations. They also demonstrate that purely situational analyses of discourse are insufficient as attempts to explain expertise. The four studies are presented as individual chapters: (1) Expertise as Mediated Collaborative Activity--a preliminary theoretical framework for the study of expertise as mediated collaborative activity; (2) The Tensions of Judging: Handling Cases of Driving under the Influence of Alcohol in Finland and California--a cross-cultural analysis of judicial expertise highlighting the multi-voicedness and internal tensions of expert work using tape recorded courtroom discourse; (3) Coordination, Cooperation, and Communication in Courts: Expansive Transitions in Legal Work--an analysis of expertise in a complex case of civil litigation in another California court setting using a court reporter's official transcripts of sidebar discussions as data on disturbances and innovations in the trial interactions; and (4) Twisting the Scripts: Heterogeneity and Shared Cognition in Multi-Professional Medical Teams--an analysis of expertise in multi-professional medical teams working in Finnish health centers which examines audio- and videotape recordings of their interactions using a three-pronged model of coordination, cooperation, and communication. Most of the 173 references listed are in English. (ALF)