In an assessment of software process improvement (SPI) in 15 software small- and –medium-sized enterprises (software SMEs), we applied the broad spectrum of software specific and system context processes in ISO/IEC 12207 to the task of examining SPI in practice. Using the data collected in the study, we developed a four-tiered pyramidal hierarchy of SPI for software SMEs, with processes in the higher tiers undergoing SPI in more companies than processes on lower level tiers. The development of the hierarchy of SPI activities for software SMEs can facilitate future evolutions of process maturity reference frameworks, such as ISO/IEC 15504, in better supporting software development in software SMEs. Furthermore, the findings extend our body of knowledge concerning the practice of SPI in software SMEs, a large and vital sector of the software development community that has largely avoided the implementation of established process maturity and software quality management standards.