Portland Brownstone found widespread use as a building stone throughout the northeastern USA, especially during the late 19th century. It is present in numerous historical civil and religious buildings and is of major importance in heritage conservation efforts in this part of the country. In this monograph, a review of what is known about Portland Brownstone is presented, from its usage and reported degradation to a scientific analysis of the mechanisms that lead to its degradation in the field. The concepts presented regarding degradation mechanisms can also be extended to other, similar building stones. This monograph can be used as a tool and a reference for conservators and conservation scientists alike.