Assembly, the putting together of parts to produce a useful product, is intuitively understood and easily discussed. Yet assembly is difficult to define in a way that provides useful measures of how well we and our machines are doing. We seek the kind of guidance provided by thermodynamics regarding work or information theory in communications and control. The paper reviews a variety of approaches ranging from practical time and motion study methodologies and econometric measures to more theoretical structures. We ask whether a science of assembly is possible and of what use it would be.