Project Portfolio Management (PPM) has the potential to bring considerable benefits to organisations. Although PPM should improve project success, not all project management based working organisations have fully adopted PPM as part of their way of working. The question that arises here is ‘why?’. The concept of PPM Adoption has not received much academic attention so far. A positive link between PPM Adoption and portfolio success has been found by a few authors, but the circumstances under which PPM should be adopted is yet considered a research gap. This research aims to address this research gap by determining how PPM Adoption is influenced. This study aims to answer the research question ? How is the adoption by organisational bodies of Project Portfolio Management influenced? ? This question is split into three sub?questions focussing respectively on a deeper study into the process of PPM Adoption, the identification and validation of factors that influence PPM Adoption, and how and under which circumstances each of these factors influences PPM Adoption. This research project is divided into (1) an explorative phase, comprising literature study and 8 expert interviews, enabling the construction of a conceptual model, and (2) a validation phase following a multiple?case holistic design case study approach with 15 relatively small case studies with a 1,5 hour interview per case at their core, in various organisations in The Netherlands. The studied organisations can roughly be divided into one third (semi?) governmental, one third financial services and one third industry, technology and/or professional services. PPM is an ambiguous term. The definition used in this report stresses the aim of PPM for selecting and prioritising projects in the portfolio. Through literature study and explorative expert interviews it has been determined that PPM Adoption is not a choice between taking up and not taking up PPM. It rather is the movement along a spectrum between the extremes of intuitive and highly formalised project selection and prioritisation. Through study of literature on adoption of innovations and on PPM and its benefits, complemented by the explorative expert interviews, nine factors have been identified that influence PPM Adoption. These factors are categorised as conditions for and drivers of PPM Adoption. The conditions are Portfolio complexity; Organisational culture; PPM Gap size (inverse); and Relative resource scarcity. The drivers are Alternative organisational priorities (inverse); Desire for better information transparency; Need for better predictability of company results; Desire for project success rate improvement; and Desire for portfolio rationalisation. Based on mainly cross?case synthesis, considering multiple aspects of the collected data about these nine factors (conditions and drivers), conclusions have been derived about the influence of these nine factors on PPM Adoption. Besides the frequencies of these factors, also the phase of the interview in which the factor was mentioned and the additional comments by the interviewees have been included in the determination of an overall interviewee? and factor?specific judgement of the overall influence of this factor on each case’s PPM Adoption. Quantification of these judgements provides overall per?factor indications of their average influence on PPM Adoption and its standard deviation. Bringing all these components together, the following conclusions have been drawn about the individual factors. Of the four conditional factors, only ‘relative resource scarcity’ has been found to form an important condition for PPM Adoption. Of the driving factors, ‘desire for information transparency’ is an important driver for PPM Adoption, regardless of circumstances. Also ‘desire for portfolio rationalisation’ is frequently a driver, but often tacitly. This means that organisations are not aware of this desire, until they discover that this desire is fulfilled by PPM Adoption, which is then obviously driven by other factors. The influence of ‘need for better predictability of company results’ can be influential, but this is strongly dependent on circumstances, like how the organisational body hierarchically reports to another entity. The ‘desire for project success rate improvement’ can be a reason for adopting PPM, but conversely PPM is not always adopted to answer this desire. This study contributes to the extant body of literature by contributing to the bridging of the research gap in influences on PPM Adoption. Among its principal contributions are a conceptual model for researching the influences on PPM Adoption, a consideration of influencing factors that potentially provide wider application in the research on adoption of other organisational innovations, and ? more generally ? the contribution of empirical evidence to the extant body of literature on PPM through 15 case studies in organisational bodies that practice PPM. The managerial implications of this research comprise the perspective on PPM Adoption as movement along a continuum, rather than a one?shot action, and the understanding that PPM is often adopted reactively, in response to an emerging issue like a growing scarcity of resources, while proactive adoption of PPM allows enjoyment of many more benefits than solely the reduction of organisational pain.