This book presents a summary of the topic of supercooling during crystallization in condensed films. While recent findings are mainly published in English, the foundational classical results were originally published in Russian, with limited accessibility to general readers. The present work is based on a 2019 Ukrainian monograph, "Temperature Stability of the Supercooled Liquid Phase in Condensed Films," which has been extensively revised and expanded. The book includes a detailed analysis of the thermodynamics of supercooled fluids, with updated and expanded sections. Additionally, new results on the supercooling of indium-lead (In-Pb) alloys in contact with amorphous molybdenum and fusible metals in contact with nanocrystalline layers are presented. These layers occupy a middle ground between amorphous (carbon, molybdenum, as-deposited germanium films) and polycrystalline (copper, silver, aluminum) substrates. The book gives particular attention to the peculiarities of contracted geometry conditions, which are natural for multilayered structures and can occur through fusible component segregation at grain boundaries. The analysis of new data has prompted a rethinking of the role of the more refractory layer's microstructure on the crystallization processes of metastable melts. The book includes a thorough discussion of these findings, highlighting the crucial role of the microstructure in the crystallization process. This book is a valuable resource for researchers and students interested in crystallization in thin-film metallic systems. This comprehensive study provides a detailed and authoritative analysis of the thermodynamics of supercooled fluids, and the impact of microstructure on the crystallization processes of metastable melts, making it an essential addition to any academic library.