Aggregation in cohesive powders is a well known behaviour and it is responsible of both poor fluidization of cohesive powders and of a reduced effect of aeration in the promotion of silo discharge. The size of the aggregates is determinant in the definition of the fluid dynamic behaviour of these powders. The aggregate size can be estimated starting from the measurement of flow properties, and it can be used to predict the solids discharge rate in the aerated discharge of cohesive powders and to calculate the minimum fluidization velocity in vibrated fluidization of similar powders. In the former case the aggregate size of the aggregate is the one for which the weight of the aggregate equals the cohesive forces keeping the aggregate with the solids bulk. In the latter case the approach is similar but the role of the weight is played by the aggregate inertia which depends on the vibrational acceleration rather than on the gravitational acceleration. It is, therefore, shown that aggregation can reasonably be estimated by the use of conventional techniques and that, if correctly accounted for, it allows to predict the fluid dynamics of cohesive powders.