Abstract: Nonreciprocity of thermal metamaterials has significant application prospects in isolation protection, unidirectional transmission, and energy harvesting. However, due to the inherent isotropic diffusion law of heat flow, it is extremely difficult to achieve nonreciprocity of heat transfer. This review presents the recent developments in thermal nonreciprocity and explores the fundamental theories, which underpin the design of nonreciprocal thermal metamaterials, i.e., the Onsager reciprocity theorem. Next, three methods for achieving nonreciprocal metamaterials in the thermal field are elucidated, namely, nonlinearity, spatiotemporal modulation, and angular momentum bias, and the applications of nonreciprocal thermal metamaterials are outlined. We also discuss nonreciprocal thermal radiation. Moreover, the potential applications of nonreciprocity to other Laplacian physical fields are discussed. Finally, the prospects for advancing nonreciprocal thermal metamaterials are highlighted, including developments in device design and manufacturing techniques and machine learning-assisted material design.