In this review, we examine recent theoretical and experimental investigations on 3D and layered hybrid perovskites (HOP), that combine spectroscopic studies, classical solid-state physics concepts and density functional theory (DFT) simulations to understand their exceptional photovoltaic and optoelectronic properties. It allows defining a new class of semiconductors, where the pseudocubic high temperature perovskite reference structure plays a central role for 3D HOP [1]. A general symmetry analysis of electronic Bloch states, lattice strain, molecular rotations and optical phonons yield new insight into the influence of lattice distortions, including loss of inversion symmetry, as well as spin-orbit coupling. Electronic band folding and degeneracy, phase transitions, effective masses, carrier collision processes and optical absorption are analyzed. Thermally activated molecular rotations and disorder, are important for room temperature screened excitonic properties of 3D HOP. Quantum and dielectric confinements in layered HOP are quantitatively determined, using a new DFT method [2]. The theoretical concepts are compared to recent experimental investigations on the excitonic properties of HOP [3]. [1] J. Even, et al, J. Phys. Chem. Lett. 4, 2999 (2013), J. Phys. Chem. C, 118, 11566 (2014), J. Phys. Chem. Lett., 6, 2238 (2015) [2] J. Even, et al, Phys. Rev., B 86, 205301 (2012), Chem. Phys. Chem. 15, 3673 (2014) [3] H.-H. Fang, et al, Adv. Func. Mat., 25, 2378, (2015), G. Lanty, et al, J. Phys. Chem. Lett. 5, 3958 (2014)