Small particles and organisms in the aquatic environment can be represented in model simulations in a very direct way by ‘model particles’. The behaviour of one model particle is a close approximation of that of a real particle; a set of model particles corresponds to a random sample of the corresponding set of real particles. A description is given of the characteristics of the model particles and the mathematical models in which they figure. Two types of sub-gird modelling are briefly described and compared. Examples of applications and comparison between measurements and simulations are presented.