AbstractCrystallographic structures of decagonal quasicrystals (d‐QCs) are traditionally described as a periodic stacking of atomic planes with quasiperiodic in‐plane atomic order, so that d‐QCs are considered to be two‐dimensional (2D) quasicrystals, whereas they are periodic crystals in the third dimension. Similar stacked‐layer structures are observed also in the periodic decagonal approximant phases. In this review paper, we consider the dimensionality of the chemical bonding network in the d‐QCs and their approximants on the basis of electrical resistivity. By comparing the anisotropic resistivity along the stacking‐ and the in‐plane directions of a series of decagonal approximants with different numbers of atomic layers within one periodicity unit (the two‐layer Y‐Al‐Co‐Ni, the four‐layer o‐Al13Co4, Al13Fe4 and Al13(Fe,Ni)4, and the six‐layer Al4(Cr,Fe) and T‐Al3(Mn,Fe)) and of a two‐layer d‐Al‐Co‐Ni decagonal quasicrystal, we show that universally, the stacking direction perpendicular to the atomic planes is always the most conducting one. Since the in‐plane electrical resistivities are of the same order of magnitude as the resistivity along the stacking direction, this confirms the 3D character of the investigated solids. The stacked‐layer description in terms of 2D atomic planes should therefore be regarded as a convenient geometrical approach to describe the complex structures of the d‐QCs and their approximants, whereas their physical properties are those of true 3D solids.