Covalent organic frameworks (COFs) are crystalline, porous networks built from organic linkers via covalent bond formation. Most COFs to date are layered materials composed of atomically thin two-dimensional (2D) sheets, which create stacked crystals via non-bonding interactions. Commonly, these materials are referred as 2D COFs, a somewhat misleading nomenclature as most representatives are layered 3D bulk materials. Although we will show that in most layered COFs there is no periodicity normal to the 2D crystal plane and thus a common definition of a 2D periodic material (translational symmetry only in two dimensions) fits, we will, following the traditional nomenclature of related layered materials, employ here the term layered COFs (LCOFs).