Systems engineering has been plagued by the problem of how to separate a system from its environment or context, in particular from its social context. We propose to include anything in the system that is necessary for performing its intended function and that may be the object of design. For certain engineering systems, such as civil aviation systems, this implies that human agents and social institutions have to be taken as integral parts of these systems. These ‘socio-technical’ systems are of a hybrid nature because they are constituted by different kinds of elements, intentional and non-intentional: social institutions, human agents and technical artefacts. This paper analyses two different roles that human agents, as elements of socio-technical systems, may play with regard to technical artefacts. Furthermore, it discusses some conceptual problems concerning the modelling of socio-technical systems that are due to the hybrid nature of these systems. Copyright © 2006 John Wiley & Sons, Ltd.