Software Process Improvement initiatives have been implemented by many companies in order to achieve quality of products and practices. Many models like CMMI and IDEAL have been adopted as a means to gain competitive advantages among competitors and trustworthiness of customers. Although these models have proved successful results, the inherent characteristics of SMEs make it difficult and in many cases unfeasible to implement such models, without meaning that those companies are less capable of producing quality products or adopting well-defined practices. This research presents a model to guide Software Process Improvement (SPI) according to the context of Small and Medium Enterprises (SMEs) using the Open Maturity Model (OMM) elaborated by the QualiPSo consortium. Different literature reviews, a survey and a focus group were done in order to build the theoretical foundations that support the structure of the proposed SPI model, validate the findings and evaluate the resulting SPI model. SPI in SMEs, CMMI implementations in SMEs, issues affecting SPI programs, and Knowledge Management approaches supporting improvement of software processes and practices are the four main pillars of the theoretical background supporting the design of the proposed SPI model, which is the major outcome of this thesis project and is aimed to guide SPI programs considering OMM assessment results. The proposed SPI model was subject of a preliminary evaluation with researchers and a validation with practitioners. The results have confirmed that SPI outcomes are more valuable when business requirements and SPI objectives are chosen to drive the improvement actions. The importance of organizational awareness and the value of knowledge management strategies to mitigate potential problems faced when implementing SPI has also been highlighted when evaluating the model.