This module presents current conceptual models of several MCS types and provides explanations for the structures and behavior of MCSs based on the physical processes underlying their evolution. An understanding of the physical processes and conceptual models of MCSs will help forecasters to predict the most likely locations of severe weather within existing systems and to forecast the longevity, areal extent, and path of the system. Accompanied by conceptual animations, numerical simulations, and case studies, Mesoscale Convective Systems: Squall Lines and Bow Echoes presents strategies with which the forecaster can identify the potential for long-lived MCSs and attendant severe weather.