Abstract: The developments of the notions of sustainability and systems thinking have similar triggers and are complementary in their purpose. Both have evolved in response to an inability of reductionist approaches to achieve desired outcomes when dealing with messy and complex issues. Both concepts call for holistic approaches to research, development, monitoring and evaluation. Indeed, systems approaches have to be integral to sustainability investigations if the sustainability concept is to have coherent meaning. The challenge is how to establish, in applied studies of sustainability, the appropriate boundaries of the system. This challenge arises because of the multiplicity of stakeholder perspectives, together with the multiplicity of spatial and temporal scales that might be considered. Different strands of systems thinking inform the process in different ways. Within hard systems thinking, boundary issues tend to be assumed within a delimiting framework that ignores the social dimension. Hard systems thinking is therefore generally supportive of quantitative modelling undertaken by 'experts'. Within soft systems thinking, definition of the elements, interactions and boundaries of the system are recognised as essentially being an artefact of the modeller to try and make sense of a chaotic world. Hence, there is explicit recognition that the system is socially constructed and that system improvement may be viewed differently according to the perspective of different stakeholders. Soft systems thinking recognises the fallibility of the 'expert' and supports discussion in search of consensus. Critical systems thinking goes considerably further in attempting to make explicit the power relationships that drive specific interventions and assist the inclusion of the boundary issues of those whose perspectives are ignored in a sustainability plan. At an operational level the principle of subsidiarity, whereby decisions are made as close as possible to those who are affected, can be considered one guiding principle. However, considerable challenges remain as to how those who are affected can be empowered in terms of achieving the capability to shape and implement sustainability plans. Further development of linkages between critical systems thinking, capability theory of development as well as sustainable livelihood frameworks appears as one way that this might occur. Participatory simulation approaches can also help in co-learning and development of capability among stakeholders.