The methodology of complex activity (MCA) is a system of formal models that, from a systems science perspective, generalizes nontrivial human activity and the operation of enterprises and complex (sociotechnical) systems. Complex activity (CA) is defined as one with a nontrivial internal structure and with multiple and/or changing actors/players, methods, and roles of the subject matter of activity in its relevant context. A dialectically coupled pair – a complex activity and a sociotechnical system that is a complex player in activity – is the subject matter of MCA.