In his ‘Could Extended Objects Be Made Out of Simple Parts?: An Argument for “Atomless Gunk’’, Dean Zimmerman defends the claim that no physical object has a complete decomposition into simples but instead has among its parts a piece of ‘atomless gunk’ His argument for this claim rests in part upon a theory of the impenetrability of physical objects. In that theory, Zimmerman distinguishes ‘[t]he sort of impenetrability that is a part of the concept of’ a physical object from ‘a power to resist the pressure of other objects’. He says that impenetrability is but ‘an essential inability’ of two physical objects to occupy one and the same region of space, an essential inability that is not a power of one physical object to exert repulsive forces on others in its vicinity. I argue that this theory of impenetrability is false and that Zimmerman's argument for the existence of ‘atomless gunk’ fails.