Thanks to recent improvements in electronics, conductive materials and energy storage capabilities, we are now able to embed small circuits but also ultra-mobile PCs (UMPC) into clothes. These devices are able to sense, process and communicate information between the user and his/her context. During the first research phase, we have developed a set of wearable applications that aimed at assisting the user in various situations such as the teleoperation of a blimp, or the pedestrian navigation in an unknown indoor environment. Research on both of these experiments can be classified in three categories: First the multimodal interaction between the users, his/her wearable interface, and the environment. Second, the context sensing such as the location of the user. Third, the wireless communication between the embedded system and the surrounding or remote devices. Although the resulting functionalities were satisfying as expected, both systems implied the use of cumbersome equipment such as a Head-Mounted Display (HMD), or a UMPC, and thus were too restrictive. With the experience gained from these experiments, we have decided to pursue a different approach. Indeed, in order to develop a truly wearable system, we have focused our research on the design and development of wearable modules based on small 8-bit microcontrollers. In our vision, one module equals one functionality. The advantages are threefold: First, the applications are virtually unlimited as the wearable modules can be attached or removed on the fly. Second, the user only wears what is needed, thus eliminating the superfluous hardware. Third, compared to commercial wearable products, the user can change the garment without changing the functionalities, or on the opposite, s/he can change the functionalities without changing the garment. A special attention has been put in the design of magnetic connectors in order to facilitate the placement of the modules on the garment. Compared to alternative approaches such as the lilypad from Leah Buechley, our modules do not need to be sewn to the garment and thus can very easily be relocated or removed when washing the garment for example. The decision to opt for a wired bus has been motivated by several factors. Indeed, not only the modules consume less power, but also they are smaller and lighter as they don't need wireless functionalities, nor individual batteries. We have developed specific applications in order to demonstrate and evaluate our modular approach. The feedback collected from test users and the results of various evaluation techniques showed us that our system is comfortable, attractive and perceived as flexible and creative. We believe that this kind of approach may not only foster the involvement of the user in wearable computing applications, but it can also be seen as a rapid prototyping platform consisting of a set of tools that facilitate the development of wearable applications.