Metal-halide perovskite materials are crystalline semiconductors that can be processed at room temperature using solution-processible deposition techniques. In only a few years, perovskite-based solar cell efficiencies have seen a huge increase from 3 % to 22.1 %. These direct-bandgap materials are potential candidates for other optoelectronic device applications such as photodetectors, light-emitting transistors, and light-emitting diodes. In this Review, we present the current state-of-the-art in the research and development of perovskite light-emitting diodes (PeLEDs) based on metal halide perovskite semiconductors with an emphasis on size of crystallites and its effects on optical properties, device architectures, and PeLED performance parameters. A uniform pinhole-free morphology with small grain size is essential for high-performance PeLEDs. For this purpose, a p-i-n type device architecture with a p-type poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer was found to be more suitable than the n-i-p type. In PeLEDs based on bulk-phase perovskites, the average size of the crystallites is in the range 100–500 nm. The efficiency can be improved further by using perovskite nanoparticles with size <100 nm. Color of the emitted light from these materials can be tuned over a wide range, that is, from NIR (775 nm) to near-UV (410 nm), simply by changing the halide ion and the stoichiometric ratio between halide ions in the mixed-halide-type perovskites. Change of stoichiometry also affects the structural properties and the final film morphology, which in turn influences the radiative and non-radiative recombination rates of the charge carriers and hence the device performance. Furthermore, we also provide recent developments in PeLEDs based on inorganic perovskite nanocrystals that complement the efforts being made in hybrid PeLEDs. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim