We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organicinorganic compounds MA(2)CdX(4) (MA = CH3NH3; X = Cl, Br, I). MA(2)CdI(4) is a new compound, whereas, for MA(2)CdCl(4) and MA(2)CdBr(4), structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MA(2)CdX(4) solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MA(2)CdCl(4) forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MA(2)CdBr(4) and MA(2)CdI(4) adopt 0D K2SO4-derived crystal structures based on isolated CdX4 tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MA(2)CdX(4) family impact their air stabilities, investigated for the first time in this work; MA(2)CdCl(4) is air-stable, whereas MA(2)CdBr(4) and MA(2)CdI(4) partially decompose when left in air. Optical absorption measurements suggest that MA(2)CdX(4) have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MA(2)CdX(4) yield broad peaks in the 375955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MA(2)CdX(4) following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MA(2)CdX(4). On the basis of our combined experimental and computational results, MA(2)CdX(4) and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.