School improvement and reform efforts frequently involve the adoption of a new curriculum program. This article examines the factors that influence program selection when schools have freedom to choose as opposed to having programs mandated by district, state, or national education authorities. Survey and document analysis are used to examine the self-reported reasons for program selection for 336 public and independent schools worldwide who, between 1994-2007, adopted the International Baccalaureate Middle Years Program. The analysis suggests that while issues of implementation, marketing, branding, and the support of an external agency for quality control and professional development are acknowledged by schools as influencing program selection, the most important determinant of International Baccalaureate Middle Years Program selection was a perceived match between the existing school philosophy and program elements that supported it.