统计力学（Statistical mechanics）是一个以波兹曼等人提出以最大熵度理论为基础，借由配分函数 将有大量组成成分（通常为分子）系统中微观物理状态（例如：动能、位能）与宏观物理量统计规律 （例如：压力、体积、温度、热力学函数、状态方程序等）链接起来的科学。如气体分子系统中的压力、体积、温度。易辛模型中磁性物质系统的总磁矩、相变温度、和相变指数。

通常可分为平衡态统计力学，与非平衡态统计力学。其中以平衡态统计力学的成果较为完整，而非平衡态统计力学至今也在发展中。统计物理其中有许多理论影响着其他的学门，如信息论中的信息熵。化学中的化学反应、耗散结构。和发展中的经济物理学这些学门当中都可看出统计力学研究线性与非线性等复杂系统中的成果。

Statistical mechanics is a branch of theoretical physics that studies, using probability theory, the average behaviour of a mechanical system where the state of the system is uncertain.

A common use of statistical mechanics is in explaining the thermodynamic behaviour of large systems. This branch of statistical mechanics which treats and extends classical thermodynamics is known as statistical thermodynamics or equilibrium statistical mechanics. Microscopic mechanical laws do not contain concepts such as temperature, heat, or entropy; however, statistical mechanics shows how these concepts arise from the natural uncertainty about the state of a system when that system is prepared in practice. The benefit of using statistical mechanics is that it provides exact methods to connect thermodynamic quantities (such as heat capacity) to microscopic behaviour, whereas in classical thermodynamics the only available option would be to just measure and tabulate such quantities for various materials. Statistical mechanics also makes it possible to extend the laws of thermodynamics to cases which are not considered in classical thermodynamics, such as microscopic systems and other mechanical systems with few degrees of freedom.