Software Quality and productivity are critical factors for the sucess of software enterprises around the world. For this reason they become in two of the main goals of software development enterprises like -SMEs-, among others, due to the fact that this kind of industries represent most of the software industry in the world. These factors have been taken to the development of different studies in order to strengthen the competitiveness in this kind of enterprises. A strategy to increase the competitiveness in the SMEs, is the software process improvement -SPI-. However, carry out a SPI project in any software enterprise, is a hight risk initiative, whose success can not be assure by a model or a methodology. A doubt cames up ¿What does determine the success of a SPI project in a SME? Starting from the success and failure experiences obteined from the application of a SPI projects in four SMEs, this article gathers the desicive factors for the succes of the SPI projects in the context of micro, small and medium enterprises. The goal is to present the key factores that a SME must consider when it pretends to iniciative a SPI project.