The paper revisits some of the well-known models in the mechanics of structurally heterogeneous media for the purpose of analysing their suitability to describe properties of nanocomposites and their mechanical behaviour. It also presents a new multi-component model for predicting the mechanical properties of micro- and nanocomposites reinforced either by whiskerising the microfibres or by bristlising the nanowires. The mathematical formulation of the model is based on using the Muskhelishvili complex potentials for each domain occupied by a separate component. As an example, the effective elastic constants are computed for fibrous composites with four different densities of whiskerisation. It is shown that the increase in the number of bristles per unit surface of the fibres gives a very strong rise to the value of Young’s modulus. However, the shear modulus, being the driving parameter for the strength estimation of the entire composition, is less sensitive to this factor.