Not only might one comfortably ask "why one would write a book on this subject?", but one might further wonder why anyone would resurrect it from the past. My reasons revolve around the original reasons for writing the monograph in the first place. I have always regarded the virial theorem as extremely powerful in understanding problems of stellar astrophysics, but I have also found it to be poorly understood by many who study the subject. While it is obvious that the theorem has not changed in the quarter-century that has passed since I first wrote the monograph, pressures on curricula have reduced the exposure of students to the theorem even below that of the mid 20th century. So it does not seem unreasonable that I make it available to any who might learn from it. I would only ask that should readers find it helpful in their research, that they make the proper attribution should they employ its contents.