Collapsible filter assembly includes a filter medium element and a frame at least partially surrounding the filter medium element. The frame and filter media element are configured to vary in at least two dimensions by folding or unfolding. In at least on example, the filter medium element is rectangularly pleated or round pleated. Upon removal of the filter assembly from packaging and other restraints, the filter assembly may naturally expand with regard to footprint and volume, with some manual assistance being expected of a typical user in unfolding from an area-reduced configuration to an area-expanded configuration, and from a volumetrically reduced configuration to a volumetrically expanded configuration.