This Software industry is a very significant cog in today’s economic landscape. The majority of these organizations mainly comprise small and medium enterprises (SMEs). These enterprises aim to benefit from Software Process Improvement (SPI) activities in producing high-quality products and services in competitive business environments. For this purpose, SPI models have been developed for specific SME characteristics. In this study, we performed a systematic literature review to analyze the characteristics of these SPI models, the challenges of performing SPI activities in SMEs, and the critical success factors for SPI activities. In this context, 61 articles published from 2007 to 2020 were examined, as a result, 28 SPI models used in SMEs were found out. In addition, ten different situations that make SPI implementations challenging in SMEs and seven factors that affect the success of SPI studies in SMEs were reported.