The acquisition of goods, works and services by public sector organizations accounts for approximately 17% in the global economy. By strategically leveraging this procurement demand and integrating sustainability objectives, the public sector can drive the transition toward more sustainable supply markets, while also creating broader value for society at the contract level. However, despite growing policy support and increasing awareness of its potential, the implementation of Sustainable Public Procurement (SPP) remains limited. The first two studies in this dissertation investigate what motivates SPP efforts. The first study shows that simply addressing funding and knowledge constraints may not be as effective as is often suggested. The second study illustrates that, rather, SPP efforts can be understood using a legitimacy theory framework that coherently considers the wider organizational and institutional dynamics that motivate procurement behaviour. The third study of this dissertation investigates the implementation of SPP in practice. It first clarifies the specific issues that arise in pursuit of broad social value creation, which consists of multiple sub-objectives. Then, it explains how to arrange the engagement with private and/or civil actors, resolving these issues.