The guiding feature of this book is the typology of male friendships.  Dr. Greif dispels the myth that men don't have friends, showing that men must have must, trust, just and rust friends.  A must friend is the best friend a man absolutely must call with earthshaking news.  A trust friend is liked and trusted by not necessarily held as close as a must frien.  Just friends are casual acquaintances, while rust friends have a long history together and can drift in and out of each other's lives, essentially picking up where they last left off.  Understanding the role each of these types of friends plays across men's lives reveals fascinating developmental patterns, such as how men cope with stress and conflict and how they make and maintain friendships.