Growth of Web-based applications has drawn a great number of diverse stakeholders and specialists into the information systems development (ISD) practice. Marketing, strategy, and graphic design professionals have joined technical developers, business managers, and users in the development of Web-based applications. Often, these specialists work for different organizations with distinct histories and cultures. A longitudinal, qualitative field study of a Web-based application development project was undertaken to develop an in-depth understanding of the collaborative practices that unfold among diverse professionals on ISD projects. The paper proposes that multiparty collaborative practice can be understood as constituting a “collective reflection-in-action” cycle through which an information systems (IS) design emerges as a result of agents producing, sharing, and reflecting on explicit objects. Depending on their control over the various economic and cultural (intellectual) resources brought to the project and developed on the project, agents influence the design in distinctive ways. They use this control to either “add to,” “ignore,” or “challenge” the work produced by others. Which of these modes of collective reflection-in-action are enacted on the project influences whose expertise will be reflected in the final design. Implications for the study of boundary objects, multiparty collaboration, and organizational learning in contemporary ISD are drawn.