The term “transit desert” is a new concept that looks at the gap between level of transit service (supply) and needs of a particular population (demand). These populations are often referred to as “transit dependent,” people that are too young, too old, or too poor or who are physically unable to drive. “Transit deserts” in this case are defined as areas that lack adequate public transit service given areas containing populations that are deemed transit-dependent. This study aims to analyze and establish a clear method for calculating and quantifying gaps between transit demand and supply using Geographic Information Systems (GIS). The study looks at four major U.S. cities: Charlotte, North Carolina; Chicago, Illinois; Cincinnati, Ohio; and Portland, Oregon. Transit deserts often occur in neighborhoods surrounding historic downtowns; however, exceptions occur in very isolated rural areas.