Mesoscale convective systems (MCSs) are responsible for ~60% of summer rainfall in the U.S. Great Plains and 50-60% of tropical rainfall globally. Deficiency in representing MCSs contributes importantly to climate model biases in simulating precipitation and its diurnal variability over the central U.S. and tropical circulation, with important implications to modeling the regional and global water cycles. In the past decades, observed increases in springtime total and extreme rainfall in the central U.S. have been dominated by increased frequency and intensity of long-lasting MCSs. Understanding the environmental conditions producing long-lived MCSs is therefore a priority in determining how the characteristics of precipitation may change in the future. Regional and global variable resolution models are being used to perform convection permitting simulations of MCSs and their interactions with the large-scale environment. The large-scale and mesoscale ingredients identified from reanalysis and simulations provide a framework for understanding MCSs and modeling their impacts on hydrologic extremes.