Aerosol-cloud interactions refer to the group of atmospheric processes by which aerosols influence cloud properties, and sometimes also processes by which clouds affect aerosols. The effect of these atmospheric processes on Earth’s radiative balance is potentially large, but uncertain. When combined with uncertainties in aerosol concentrations that result from emissions and aerosol processes, the uncertainty in aerosol-cloud interactions dominates the overall uncertainty in our knowledge of radiative forcing of Earth’s climate. Aerosols affect clouds primarily by changing the number of cloud condensation and ice nuclei, “indirect effects,” and sometimes also the temperature of the cloud, “semi-direct effects.” Changes in cloud processes in response to aerosol-cloud interactions may cause significant adjustments to cloud macrophysical properties such as coverage and condensate amount. Aerosol-cloud interaction research focuses on understanding the atmospheric processes at work, mainly by analyzing observation data, performing laboratory experiments, and building models to simulate how aerosols influence clouds. In this review, we outline the relevant atmospheric science and highlight some promising techniques that have been applied recently to better understand aerosol-cloud interactions and their implications for radiative balance, such as Gaussian process emulation. This chapter is intended to provide background to subsequent chapters in this series of monographs and as an introduction for graduate students to current research in the field of aerosol-cloud interactions.