The general adoption of the International System of Units (SI) in engineering as well as in science makes this the time to relinquish the slug, an engineering unit of mass. Seldom has a convenient unit proved so contentious. This note sets out salient points in the controversy, and indicates basic issues which could scarcely have been appreciated universally at the time.

In essence the slug is the unit of mass that goes with the pound force or the pound weight, just as the poundal is the force unit that goes with the pound-mass. There has, however, been a great deal of argument as to whether it is a true derived unit or simply a fundamental unit in disguise, and about how to realise it in practice: in particular, if unit force is defined or established by reference to the gravitational force on the pound-mass, there arises immediately the question of the appropriate value to use for g, i.e. should it toe the local value, the value at a specified place, or some other agreed standard acceleration?