This chapter seeks to demonstrate how a relational understanding of sustainability competencies can assist a sustainable future. The author discusses how better understanding of relationships within and between competencies helps build knowledge, skills, values, and attitudes in sustainable development. While all eight sustainability competencies identified by Redman and Wiek (2021) are vital, one competency, systems-thinking, is identified as a meta competence that supports development of all other competencies by understanding interdependencies between them. The current lack of systems thinking competence is a key obstacle to sustainable development and evident in current mismanagement of many wicked environmental crises such as economic shocks, environmental vulnerabilities, and societal disruptions. The chapter presents a theoretical model that can help build systems thinking competence and thereby improve management efforts. This model distinguishes between systems thinking complexity, coupled cognitive and cultural competence, and systems thinking organizational properties. Regarding systems thinking complexity, the model offers a novel account of within relations of self-organization and between relations in sociocultural organization. Regarding systems thinking organizational properties the model identifies within and between relationships of structures, processes, functions, and contents. The utility of the model for advancing systems thinking competency is illustrated through application to two axes of sustainable development practice: the axis of local to global scales and the axis of incremental reform to radical transformation. Limitations and prospects for future research are outlined.