The hospital health sciences library of today that reaches out to the world knowledge base through electronic networks bears little resemblance to its forebears. Yet to understand the challenges and future directions of the hospital library it is necessary to examine how it began and how it has evolved in more than 200 years. This paper identifies five developmental periods in which major strides were made: the colonial years through the 19th century; World War I to the Great Depression; World War II and the 1950s; the 1960s--the Great Society and the Medical Library Assistance Act; and the 1970s, an era of growth for hospital libraries.