Metal nanomaterials are of great importance in the field of heterogeneous catalysis. In general, the catalytic performances of metal nanomaterials are determined by the structures. However, far from being static, dynamic reconstruction of metal nanomaterials constantly occurs in reactive environments, resulting in different catalytic activities. This review summarizes the latest progress of theoretical understanding of the driving forces for the structural changes. In the first part, some typical ex situ and in situ experimental observations of catalysts in reactive environments are briefly introduced, including the changes of shape, size, and alloy composition of metal or bimetallic nanomaterials. Next, we review the state-of-the-art advancement of the theoretical calculations and simulation methods to understand these experimental observations, and categorize them according to the different driving forces, for example, the oxidation and reduction effects, adsorption-induced reconstruction. Moreover, this review provides many examples for the quantitative agreement between theoretical modeling and experimental observations, which indicates the potential applications for the rational design of high-performance metal nanocatalysts in real reactions. This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Computational Materials Science