Software Process Improvement (SPI) is an approach to systematic and continuous improvement of a software producing organization's ability to produce and deliver quality software within time and budget constraints. SPI initiatives often experience difficulties and problems when the improvements are going to be implemented and institutionalized in the organization's daily practices. In this paper we discuss the learning processes that take place in SPI based on an analysis of three industrial SPI projects. A finding of the study is that an SPI project needs to pay attention to the creation of both tacit and explicit knowledge in order to improve practices. A second finding suggests that implementing and institutionalizing improvements should take its starting point at the group level and involve the development projects. A third finding suggests that sustained SPI processes require ongoing interactions between different learning processes.