Multi-agent systems exhibit highly dynamic behaviour within dynamic environments. Modelling of individual agents within such systems demands considering both evolving data structures and the control over their internal changing states. In addition, modelling of the overall system implies modelling of the agents configuration, including their ability to exchange messages as well as the ability to re-structure their formation over time. This is particularly the case with biological multi-agent systems, such as colonies of ants or bees, flocks of birds, cells, tissues etc. This talk briefly presents a formal modelling framework based on Communicating X-machines, allowing the specification of multi-agent software systems with a dynamic structure and behaviour. An abstract case study illustrates the proposed modelling approach.