Part 1 Marx's theory of the falling rate of profit: increase in the composition of capital? - definition of the composition of capital; theory of the tendency to increase; composition of capital increase faster than the rate of surplus-value? - Marx's argument; formal model with a constant real wage; formal model with an increasing real wage; Okishio's theorem - the theorem, criticisms, evaluation. Part 2 Conceptual issues in the estimation of the Marxian variables: money or labour units; non-capitalist production; non-production capital (productive labour/unproductive labour); residential housing; taxes on wages; recent criticisms of Marx's concepts of productive labour and unproductive labour. Part 3 Estimates of the Marxian variables for the postwar US economy: rate of surplus-value - composition of capital; technical composition of capital; value composition of capital; organic composition of capital; distribution of capital across industries; turnover time of capital; multiple shifts; rate of profit; comparison with Weisskopf's estimates; comparison with Wolff's estimates; estimates of the Marxian variables, 1977-87. Part 4 The decline of the conventional rate of profit: "profit squeeze" explanations; Weisskopf - "rising strength of labour"; Wolff - "slower productivity growth"; summary; Marxian explanation - Marxian theory of the conventional rate of profit; estimates of the Marxian determinants; share of profit; empirical test. Part 5 The causes of the increase of unproductive labour: detailed estimates of unproductive labour; commercial labour; financial labour; finance; insurance and real estate; supervision labour - causes of increases; effects of increase; conclusions. Part 6 Conclusion: main conclusions; further research; future trends; limits of government policies. Appendices: detailed estimates; sources and methods; assessment of bias.