Complete descriptions of the biology of all invasive macrofouling mollusks would require a tome and is beyond the scope of this book. Brief descriptions of the external and internal structures are provided to introduce the reader to the most common terminology used in describing the macrofouling bivalves and snails. The overviews examine only those biological features that are relevant to the monitoring and control of the invasive macrofouling mollusks. Description of their shell and soft parts is essential because they not only help to explain why the mollusks are pests but the descriptions are also important for distinguishing it from native species of mollusks. Some explanation of their habits and habitat is important for knowing why they can live and be so proli c in pipelines, on intake structures, in canal systems, in aqueducts, and in lakes and rivers. Knowledge of the mollusk’s reproductive potential helps to explain their biofouling nature and why they are so much more proli c than our native species that also live in fresh waters. Information on their life cycles and population dynamics are crucial for successful control of the species. The ef cient dispersal mechanisms employed by the nuisance mollusks help to explain not only why they have spread so rapidly to inland lakes and rivers across continents, but also how they can invade virtually any part of a facility or water body that are not protected. An understanding of their physiology helps to explain why some natural physical (e.g., heat) and chemical properties (e.g., salinity, pH, alkalinity, calcium levels, dissolved oxygen levels) and nutrient (e.g., total phosphorous and chlorophyll) levels can be effective control agents. Finally, knowledge of their potential impacts on ecosystems, industries, and utilities, as well as of the many human-made physical and chemical mitigants that are being used to control the mollusks, is crucial for preserving the health of our raw water supplies.