When (re)designing a work environment, tasks or functions are allocated more or less explicitly among humans and between humans and machines. After a brief review and discussion of issues related to task allocation, we argue that an important aspect to be addressed when (re)designing socio-technical systems is the systematic evaluation of the impact of allocation decisions on the overall reliability of such systems. It is contended that the cooperative dimension of such systems is one of the main elements that contribute to this reliability. This claim leads us to present a conceptual framework for modelling the human contribution to the overall reliability of complex cooperative work systems. The framework is characterized here as a set of notions, mainly regulation and shared context, used to discuss and reason about this role of humans in the error tolerance properties of such systems. These notions are demonstrated with di!erent examples derived from empirical studies of work practices in two complex cooperative work settings (air tra$c and nuclear reactor control). We then show how this conceptual framework can be used for the evaluation of allocation decisions and more generally to inform design.