This thesis aims to extend prior research on Software Process Improvement (SPI) by examining the issue of sustainment i.e. what organisational factors influence the sustainment of SPI? It uses insights from prior literature-based research on process improvement and multiple longitudinal case studies to generate a model of organisational context factors that influence SPI sustainment over a period of eleven years. The research contributions include an SPI Sustainment Research Model and SPI Sustainment Typology. In order to achieve the desired results from SPI, it is vital to pay sufficient attention to organisational contextual factors that may influence the on-going utilisation of SPI practices. This thesis does this by identifying factors and constructs that might influence SPI sustainment through the examination of prior-literature based process improvement research. Based on this analysis, three primary categories (comprising 9 variables) of influence on SPI sustainment were identified and integrated to form a theoretical SPI Sustainment Research Model, viz.: operational capabilities (individual and team based influences); capacity for change (organisational influences); and external stakeholders (external and industry influences). It is envisaged that the contextual factors within the model can be used to manage SPI implementation and utilisation, and sustain on going improvements. The model was validated by examining six longitudinal case studies of organisations that participated in a Malaysian government SPI subsidy program introduced in 2004. Changes in the presence of sustainment variables from the model were qualitatively measured, using defined empirical indicators and rating guide, and compared with SPI process capability assessment results as each organisation progressed (or otherwise) across milestones in the program, such as maturity levels. The SPI Sustainment Typology labels three major SPI outcomes by organisations in comparison to their SPI goals: SPI improvement goal achieved (Improver); existing SPI capabilities retained (Maintainer); and existing SPI capabilities diminished (Reverter). Improvers and Maintainers are considered to sustain SPI program improvements; Reverters are not. The fundamental proposition of the research is the SPI benefits can be reinforced by congruence with the organisational context in which the program operated. Achieving sustained rewards from SPI depends not only on good implementation and utilisation of a good SPI framework but also on the positive influence of important individual, organisational and external stakeholders of the program that can influence sustainment of software process improvement efforts.