Small and medium enterprises are a very important piece in the gear of the world economy. The software industry in most of countries is composed of an industrial scheme mainly of small and medium software organizations – SMEs. To strengthen this type of organizations, efficient Software Engineering practices adapted to their size and business type are necessary. The Software Engineering community in the latest decade has expressed a special interest in software process improvement (SPI) with the purpose of improving the quality and productivity of the software. Nevertheless, there is a widespread tendency to stand out that the success of SPI is only possible for large companies. In this article a systematic review of the literature on the SPI efforts carried out in SMEs is presented. The objective is to know what has been carried out and achieved about software process improvement in this type of companies.