Working in learning groups requires learners to initiate, execute, and assess their learning in collaboration with others. An important benefit of learning in groups is that learners can communicate their own thoughts with others. Learners articulate their own ideas and listen to those of their group members as a result of which their thinking can get sustained or improved for subsequent learning. This process of sharing and communicating is called reflective thinking and can be defined as thinking about, talking about, and working with multiple perspectives. These multiple perspectives come from different group members and concern the task they are working on as well as the learning processes that come along with it. Consequently, reflective thinking is a process that takes place while in action. It is reflection-in-action referring to the view that reflection must be instantaneous and meaningful during the process of task completion. Young learners often have little experience with working in groups and are more busy finishing the task than collaborating with their group members and reflecting on the task. Their reflective thinking remains implicit and in return unavailable for others to discuss. Therefore, reflective thinking and its explicitness have to be supported. This can be done through teacher instruction or peer tutoring. Besides support from teachers and fellow learners, tools can provide the necessary scaffolds for reflective thinking. It is reported that such tools preferably satisfy one or more of the following criteria. First, they display and thus make explicit reflective processes. Second, they stimulate reflective thinking by prompting the learner to do so in different phases of task completion. Third, they provide examples of reflective processes (modelling). And fourth, they provide possibilities for discourse so that multiple perspectives can become apparent. In the light of learning in groups, the tool then extends the collaborative learning situation by adding another source to think and talk about. One of the tools that can add such an external source for the purpose of reflective thinking is e- mail.Several studies have explored what e-mail can bring to a learning situation. These have shown that e-mail promotes writing and literacy skills and stimulates more personal and social communication. Moreover, e-mail seems to promote reflective writing and appropriation of writing styles. Only a few studies have linked e-mail use to reflective thinking, however. In most cases, use of e- mail is a goal in itself and is not integrated in specific learning tasks. As a result, children have difficulties finding topics to write about. Moreover, more practical problems become the focus of research. It is found that schools lack facilities, teachers and children lack computer skills and teachers have trouble organising structural e-mail contacts that don't die after the first exchange of hello's. Of course, practical problems must be attended to. But what becomes most clear from these studies is that research is needed on the integration of e-mail in domain-specific learning tasks to make the practical effort worthwhile. As schools are aiming at independent group work with their children, we need to explore how we can support reflective thinking by use of e-mail.If e-mail is to serve as a tool for reflective thinking, this entails two equally important issues. The first issue is development of an e-mail tool that fits the existing curriculum and supports reflective thinking. The development issue revolves around fitting the tool in a domain-specific collaborative learning task. Furthermore, the e-mail tool has to invite children to articulate and share their thinking within and across the group they are working in. The second issue is implementation of the tool in existing school settings. The implementation issue revolves around making the tool usable in circumstances that differ when it comes to computer facilities, time and skills available. We decided to work on both issues simultaneously. Four primary schools were selected to enter the project for the duration of one school year. The e-mail tool was designed outside the schools and tested and evaluated inside the schools during repetitive periods of prototype testing. In this paper we address the development research approach adopted. Furthermore the learning task is discussed in which the e-mail tool was embedded and the e-mail tool itself is. Relevant design decisions are compared to findings from the use of the e-mail tool in the classrooms. We conclude with an overview of findings and formulate some general directives for the design and implementation of computer tools, including WWW-tools, for reflective thinking.