Voluntary associations have been ubiquitous in our society for hundreds of years. Efforts to develop a classification scheme have often overlooked one important segment: membership-based organizations (MBOs). MBOs are created voluntarily by a group of like-minded individuals who seek to advance their interests by organizing to promote and protect a specific domain. A number have earned the reputation for operating as "special interests." To accept that notion would not be telling the full story and would overlook the many contributions they have made. A central thesis of Special Interest...