Autopoiesis is normally considered to be the systems theory in law. In this paper complexity theory is presented as an alternative systems approach. In order to position complexity theory as a plausible alternative to autopoiesis I discuss the differing understanding of boundary within each theory, and use this as a vehicle to critique autopoiesis. My critique is situated within systems theory thinking but is external to both autopoiesis and complexity theory. Because both approaches possess an understanding of boundary it provides (...) an effective tool to contrast their differences, while permitting each to be described in its own language. It is argued that complexity theory offers an approach to boundaries as contingent, emergent interfaces, which the autopoietic construction of boundary can learn from in several ways. More generally it is suggested that the complexity approach to boundaries offers lawyers engaging with systems theory a new critical perspective to assess legal constructions. (shrink)