During the mid-1950s, massive and widespread oyster mortalities, particularly in the Delaware and Chesapeake Bay areas, served as a stimulus to establish a federal oyster mortality study program. Consequently, Congress appropriated funds to the then Bureau of Commercial Fisheries (BCF) within the U.S. Department of the Interior. These appropriations enabled federal, state, and academic institutions and laboratories to conduct research and management programs. These programs, either by contract, interagency agreement, or direct grant, were designed to recognize and understand symptoms of disease, disease processes, their cause(s), and, furthermore, to assist in the development of strategies to prevent or control shellfish mortalities and the spread of disease agents. At that time, the Annapolis Laboratory of the BCF, under the leadership of James B. Engle, was housed in an apartment building in Annapolis, MD, and the disease study was staffed by one biologist, Richard Burton, and two temporary summer technicians. I was one of those technicians, and started my career with strictly on-the-job training in histological and, later, pathological methods. In 1960, the laboratory was moved to Oxford, MD, where a larger disease research and ecology program was initiated. Little was known at that time about oyster diseases, or even how to go about studying them. Standard vertebrate methods were used in the beginning, but it soon became obvious that many techniques needed modification and refinement. Because of the unique nature of invertebrate tissues and their parasites, protocols were developed by Dr. Melbourne R. Carriker for the study of oyster diseases in individual specimens and in populations. Since that time, well over one million sections of oysters and other mollusks, crustaceans, and fishes have been processed. In this second edition of the manual, new and revised histological protocols, methodologies, and techniques developed through the cooperative efforts of several laboratory staff and from information published in the scientific literature have been included to provide a more comprehensive publication. Handwritten laboratory notes, diaries, and methods collected over the past several decades have been incorporated into this manual by the authors. The result is a working manual designed and written primarily for technicians, using very practical approaches and language which are comprehensible for someone just beginning, yet detailed and accurate enough to produce professional results in the hands ofthe experienced scientists should they elect to do the work themselves.