Organic pigments consist of tiny molecular crystals with a propensity to cluster into aggregates. Such aggregation is commercially undesirable. The properties of the crystal aggregates can be investigated by the use of nitrogen adsorption isotherms. The review describes how a measure of the extent of crystal aggregation can be provided from the combined application of nitrogen isotherms and transmission electron microscopy. The application of full nitrogen isotherms to the study of crystal aggregation is also discussed.