We report theoretical studies of exciton excitation and emission in low-dimensional metal halides, (C₄N₂H₁₄X)₄SnX₆ (X = Br, I), Cs₄PbBr₆, (Ph₄P)₂SbCl₅ (Ph₄P = tetraphenylphosphonium), (C₉NH₂₀)₂SnBr₄, and (C₄N₂H₁₄)PbCl₄. These are bulk crystalline materials, in which isolated metal halide clusters/wires are separated from each other by large inorganic or organic countercations, forming 0D/1D structures. For example, in (C₉NH₂₀)₂SnBr₄, the anionic (SnBr₄)^2- clusters are separated from each other by (C₉NH₂₀)⁺ molecular countercations. The metal halides shown above all have 0D structures except (C₄N₂H₁₄)PbCl₄, which has a 1D wire structure. High photoluminescence quantum efficiencies (PLQE) (close to 100% in some cases) have been reported for the 0D metal halides. We calculated exciton excitation and emission energies in these low-dimensional metal halides; the results are in excellent agreement with the experimental values. This enables us to study the luminescence mechanisms and suggest new compounds for synthesis. We show that exciton self-trapping in the metal-halide cluster/wire leads to localized excitons, which is important for efficient radiative recombination. The relatively low PLQE (6% - 18%) in 1D (C₄N₂H₁₄)PbCl₄ is likely due to the exciton diffusion along the wire and the subsequent nonradiative recombination at defects. The large organic countercations in hybrid organic-inorganic 0D halides plays the important role of decoupling metal halide clusters (which are luminescent centers), leading to highly immobile excitons and high PLQE. On the other hand, in all inorganic 0D Cs₄PbBr₆, the significant electronic coupling between PbBr₆ clusters and the small Stokes shift should enable resonant transfer of excitation energy and the subsequent energy loss at defects, which explains the strong thermal quenching of luminescence in Cs₄PbBr₆. We further suggest that Cs₄EuX₆ (X = Br,I), which are analogs to Cs₄PbBr₆, should exhibit improved luminescent properties compared to Cs₄PbBr₆ due to stronger exciton localization at Eu²⁺ than at Pb²⁺. Indeed, recent synthesis and characterization of these compounds show high light yields under gamma-ray irradiation at room temperature. In particular, the light yield of Cs₄EuBr₆ (78,000 photons/MeV) is the highest among the self-activated scintillators.