Three-(3D) and two-dimensional (2D) layered halide perovskites are highly promising candidates for optoelectronic applications, and this has sparked new investigations of these materials from the synthetic, physicochemical and applications point of view. The 3D versions of these compounds adopt the three-dimensional ABX3 perovskite structure, which consists of a network of corner-sharing BX6 octahedra, where the B atom is a divalent metal cation (typically Ge2+, Sn2+ or Pb2+) and X is a monovalent anion (typically Cl−, Br−, I−); the A cation is selected to balance the total charge and it can be a Cs+ or a small molecular species. Such perovskites afford several important features including excellent optical properties that are tunable by controlling the chemical compositions, they exhibit ambipolar charge transport with high mobilities. Another class of materials gaining significance are the two-dimensional (2D) perovskites -a blend of perovskites with layered crystal structure- (Ruddlesden-Popper type) offer a greater synthetic versatility and allow for more specialized device implementation due to the directional nature of the crystal structure. A remarkable advantage of the 2D perovskites is that their functionality can be easily tuned by incorporating a wide array of organic cations into the 2D framework, in contrast to the 3D analogues which have limited scope for structural engineering. We also present the new homologous series, (C(NH2)3)(CH3NH3)nPbnI3n+1 (n = 1, 2, 3), of layered 2D perovskites which is different from Ruddlesden-Popper type. Structural characterization by single-crystal X-ray diffraction reveals that these compounds adopt an unprecedented structure type which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). The these 2D perovskites combine structural characteristics from both Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) structure archetypes. Compared to the more common Ruddlesden-Popper (RP) 2D perovskites, the perovskites we describe here have a different stacking motif and adopt a higher crystal symmetry.