How do firms differ, and why do they di.er even within narrowly defined industries? Using evidence from six high-tech, manufacturing industries covering a 24-year period, we show that di.erences in sales, materials, labor costs and capital across firms can largely be summarized by a single,...

We develop a parsimonious model of innovating firms rich enough to confront firm-level evidence. It captures the dynamic behavior of individual heterogeneous firms, describes the evolution of an industry with simultaneous entry and exit, and delivers a general equilibrium model of technological...