Yale has been fortunate indeed to have had Dorothy Horstmann as a member of its faculty for all but one of the last 50 years. It has had also the wisdom to take cognizance of her value as an individual and of her contributions to biomedical science and human welfare on two occasions in recent years. Her studies of poliomyelitis, hepatitis, and rubella, executed with perceptiveness, rigor and modesty, have benefited countless numbers; and for her many achievements all are in her debt. I am beholden to her colleagues for this opportunity to pay tribute to a wise and gracious friend. In casting about for a subject befitting this occasion, the thought occurred that it might be of interest to examine the contributions of some former and present members of Yale's faculty to the subject of a group of infections still endemic in all human societies, namely those caused by Streptococcus pneumoniae or the pneumococcus. The list is doubtless not exhaustive but includes such notables as Winternitz, Blake, Paul, Trask, Eaton, and Beeson of former days, as well as reflecting ongoing investigations today by Eugene Shapiro and his colleagues. In reviewing some of this earlier work, it will be my endeavor to place it in the context of contemporary understanding. In the interest of some semblance of order, the material will be examined in topical rather than in chronological order, dealing with bacteriologic and immunologic, pathogenetic, therapeutic, and prophylactic considerations in that sequence.