In activated sludge processes, several plant configurations (like plug-flow configuration of the aeration tanks, systems with selectors, contact-stabilization processes or SBR processes) impose a concentration gradient of the carbon sources to the biomass. As a consequence, the biomass grows under dynamic (unbalanced) conditions and a storage response is usually established without any necessity of other external limitations (e.g. lack of nutrients or oxygen acceptors). Under such conditions, the storage is often the main mechanism of removal of readily available substrates, even though other mechanisms can also contribute to substrate removal, depending on the microbial composition and the previous ‘history” of the biomass. In this paper the type and the extent of this dynamic response is discussed by review of experimental studies on pure cultures, mixed cultures and activated sludges and with main reference to its relevance on population dynamics in the activated sludge. Possible conceptual approaches to storage modelling are also presented, including both structured and unstructured modelling.