Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High‐performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (Cmin). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation‐dominated process by rapidly increasing the concentration over Cmin, single crystals are mainly prepared via the growth‐dominated process by keeping the concentration between solubility and Cmin, while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.

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Epitaxial growth technology has garnered widespread attention in the field of perovskite solar cells for its potential to produce high-quality crystals with preferred orientations and superior photoelectric properties. However, achieving rational epitaxial growth in the fabrication process of perovskite solar cells still remains a challenging task. A thorough understanding of the fundamental principles of epitaxial growth and an appreciation of the unique advantages of the main epitaxial strategies are crucial for the effective application of this technique in the realm of metal halide perovskites. Unfortunately, due to the lack of a comprehensive summary of the epitaxial metal halide perovskite solar cells, current researchers have insufficiently explored the application of epitaxial growth methods, leading to a limited understanding of the underlying principles and immature process development. Therefore, this review systematically summarizes the principle of epitaxial growth technologies, the main epitaxial strategies, and the latest research progresses in its application to perovskite solar cells, aiming to assist readers in better understanding and applying this technique in the context of perovskite solar cells.