Publisher Summary This chapter discusses the shell structure and age determination of the hard clam, Mercenaria mercenaria. There is an abundance of information on the growth history of individual animals stored within the shells of bivalve mollusks. Analyses of many of these records can provide long time series of individual growth, including periods of time that precede the inception of particular studies. This recording of growth, once deciphered, interpreted, and placed in the context of other time series, can provide valuable data in a wide variety of studies, from marine biological to anthropological. The hard clam, M. mercenaria, by virtue of its widespread distribution, its cultural importance and use for thousands of years, and its shell macro- and microstructure, is well suited for such studies. The shell of the adult hard clam is composed of three aragonitic layers. Proceeding from the shell interior to exterior, these layers are (1) an inner “homogenous,” or complex crossed-lamellar layer; (2) a middle “homogenous,” or complex crossed-lamellar layer; and (3) an outer composite prismatic layer.