Co-operation between human operators and autonomous machines in dynamic (not fully controlled) situations implies a need for dynamic allocation of activities between the agents, in order to adapt the human-machine system to unexpected circumstances. Dynamic allocation is a way, for example, to avoid human workload peaks. Depending on whether tasks or functions are allocated, the demands made on human-machine co-operation design are different. Task and subtask allocation assume that both the human operator and the machine (or its designer) share the same decomposition of the overall task into subtasks. Function delegation is less demanding, provided that the human operator delegates functions to the machine explicitly, and within the context of a task representation transmitted by the human. This paper discusses these principles on the basis of experimental results taken from a series of studies on human-machine cooperation in air traffic control.