Every organism is defined by a set of vital rates that evolve to enhance lifetime reproductive fitness and survival of individuals and their progeny. These traits vary due to the complex but sometimes predictable interactions between individuals, populations and their environments. Collectively, these attributes are referred to as life history traits and include age and size of maturity, longevity, clutch size, clutch frequency, and survivorship during all phases of an organism’s life cycle. A significant body of literature has been devoted to life history theory and our intent is not to plow that ground again since it is beyond the scope of this chapter. Instead, the reader is referred to seminal papers on the topic including the works of authors such as Bernardo, Cole, Congdon, Stearns, and others. Our objective in this chapter is to review the life history traits of diamond-backed terrapins (DBTs) across all subspecies (SEE TAXONOMY CHAPTER) to better understand both their unique adaptations and their vulnerabilities in the modern world. In addition, we provide analyses of geographic variation in demography and body size among DBT populations. The latter is important since some reproductive traits in turtles are strongly influenced by body size (e.g., Ryan and Lindeman 2007). A recent summary of terrapin biology is contained in Ernst and Lovich (2009) and the reader is referred there to avoid duplication and for additional information.