The rapidly increasing pervasiveness and integration of computers in human and animal society calls for a broad discipline under which this development can be studied. We argue that to design and use technology one needs to develop and use models of humans/animals and machines in all their aspects, including cognitive and memory models, but also social influence and (possibly artificial) emotions. We call this discipline Behavioural Computer Science (BCS), and propose that BCS models combine (models of) the behaviour of humans/animals with that of machines when designing ICT systems. Incorporating empirical evidence for actual human behaviour instead of relying on assumptions about rational behaviour is an important shift that we argue for. We provide a few directions for approaching this challenge, focusing on modelling of human behaviour when interacting with computer systems.