In this paper we deal with fusion functions, i.e., mappings from [0, 1] n into [0, 1]. As a generalization of the standard monotonicity and recently introduced weak monotonicity, we introduce and study the directional monotonicity of fusion functions. For distinguished fusion functions the sets of all directions in which they are increasing are determined. Moreover, in the paper the directional monotonicity of piecewise linear fusion functions is completely characterized. These results cover, among others, weighted arithmetic means, OWA operators, the Choquet, Sugeno and Shilkret integrals. 
Directional monotonicity of fusion functions