Filtration is widespread in the biological world at many levels, from cells and organs to wetland ecosystems. As a basic yet crucial process of separating solid material from a fluid medium, filtration is also ubiquitous in the engineered human world, with collected solids retained or eliminated. Although the principle is simple, filtration can be complex in natural and artificial systems to improve efficiency and prevent filter failure or clogging. Suspension feeding is a classic example of filtration in living and extinct aquatic animals, including many of the largest species of each group. Aquatic animals use modified gill rakers, teeth, or other oral or pharyngeal structures to separate organic detritus or whole organisms from water. Baleen, a novel oral tissue that evolved in whales 25–30 million years ago, is a remarkably effective filtering material that allowed mysticetes to become the largest animals that ever lived. Although exposed baleen is dead keratin, like hair and nails, it is an ever-growing material that replaces lost parts due to generative cells in the gingiva. Baleen combines rigid strength with pliant flexibility, enabling it to withstand pressures and forces of flow that would break most materials. Its unique construction as a fiber-reinforced composite keratinous matrix, with hollow horn tubes surrounded by intertubular keratin all sandwiched between two flat sheets of cortical keratin, provides an ideal biomimetic focus. Baleen withstands cracking except along its longitudinal axis, producing long, hair-like fringes as it slowly erodes. The combination of comb-like plates and their medial fibrous fringes together facilitate filtration and avoid breakage. Dynamic variable porosity based on prevailing flow, in throughput or tangential filtration, helps baleen filters to resist high forces without clogging and persist over a whale’s entire life spanning 100–200 years. In addition to baleen’s utility in traditional biomimetics—spurring creation of improved artificial filters—there are equally exciting opportunities to “complete the circle” via the opposite process: by reverse, retro, or reciprocal biomimetics. Controlled laboratory experiments and computational simulations enable testing of materials/conditions that do not exist in nature, permitting better understanding of how mysticete filtration truly operates in vivo. Traditional and reciprocal biomimetics complement and inform each other, and, in a tightening Archimedean spiral, collaborate to enhance biological understanding and product design.