The Refined Zigzag Theory (RZT) for homogeneous, laminated composite and sandwich plates is developed from a multi-scale formalism starting with the in-plane displacement field expressed as a superposition of coarse and fine contributions. The coarse displacement field is that of first-order shear-deformation theory, whereas the fine displacement field has a piecewise-linear zigzag distribution through the thickness. The resulting kinematic field provides a more realistic representation of the deformation states of transverse-shear-flexible plates than other similar theories. The condition of limiting the homogeneity of transverse-shear properties gives rise to a set of robust zigzag functions. Unlike previously used methods, which often result in anomalous conditions and nonphysical solutions, the present theory does not rely on transverse-shear-stress equilibrium constraints. For all material systems, there are no requirements for the use of transverse-shear correction factors to yield accurate results. To model homogeneous plates with the full power of zigzag kinematics, infinitesimally small perturbations of the transverse shear properties are used, thus enabling highly accurate predictions of homogeneous-plate behavior without the use of shear correction factors. The RZT predictive capabilities to model highly heterogeneous sandwich plates have been critically assessed, demonstrating its superior efficiency, accuracy, and a wide range of applicability. The present theory, which is derived from the virtual work principle, is well-suited for developing computationally efficient, C0-continuous finite elements, and is thus appropriate for the analysis and design of high-performance load-bearing aerospace structures.