This paper draws together material from Operational Research (OR) and Human Sciences (HS) conferences. It explores the need for OR to embrace the full range of knowledge available from HS and for HS, in turn, to integrate and express that knowledge in a form that OR can use. The paper discusses humans as a source of individual and collective variability in complex, socio-technical systems and exposes the challenges facing OR in advising on interventions in them. It then outlines practical approaches to those challenges based on balanced problem formulation, modelling systems ‘naturally’ and embracing uncertainty, leading towards ‘full spectrum’ analysis.