The small-scale mixing of clouds with their environment is an essential cloud process. Following an entrainment event, turbulent mixing breaks down the entrained air and homogenizes it with the cloud, covering multiple orders of magnitude in lengthscales from the entraining eddies (∼ 100 m) down to the Kolmogorov length (∼ 1 mm). The character of this process, traditionally categorized into homogeneous and inhomogeneous mixing scenarios, can affect the microphysical composition of clouds, with commensurate impacts on large-scale cloud properties such as the cloud albedo and cloud lifetime. Based on the current physical understanding of the small-scale mixing of cloudy and cloud-free air, this chapter will summarize the basic theories describing this process. By considering the wide range of involved scales, we will outline different observational and numerical approaches used to investigate this process in clouds, as well as methods to parameterize it in large-scale numerical models. Finally, we will review the impacts of the small-scale mixing process, focusing on microscale changes in the droplet size distribution as well as macroscale effects relevant to our understanding of clouds in the climate system.