Assemblage theory provides an innovative approach to analysing governance and a unique position from which to understand processes and situations of polycentrism. Its strength lies in understanding constellations that involve large numbers and multiple forms of actors and in exploring governing relations that are informal and often subject to rapid change. It is also well suited to understanding situations where there is an overlap between jurisdictions and where new governance techniques such as best practices or public–private partnerships are employed. Assemblage theorizing originates in political anthropology and geography where scholars drew on insights from social theory, relationalism, and practice theory. Today, assemblage theorizing is adopted across disciplines to study governance at different scales, from the city to regions and global processes.. This chapter provides a concise overview of assemblage theory as an analytical tool for studying governance. We start by exploring the core conceptual apparatus of assemblage theory and discuss concepts such as territorialization and socio-material relations. Laying out the basic understandings of techniques, legitimacy, and power that emerge, we turn to empirical examples to further explore these issues. We draw on the examples of the governance of two particular spaces, the oceans and cyberspace, in order to illustrate the analytical power of the approach. Both spaces involve complex territories and actor constellations. In the empirical discussion, we show how governing evolved using best practices and public–private partnerships. Both examples show how assemblage often implies de-politicization, with power relations becoming hidden.