This paper examines how the rise of big business shaped American innovation in the early 20th century. Using newly constructed data linking patents, firms, inventors, and R&D labs from 1875-1955, I disentangle three mechanisms: firm size effects, inventor sorting, and R&D organization. Exploiting variation from the Great Merger Wave, I find that increased firm scale led to disproportionate growth in innovation at the firm level, with mixed but mostly positive results at the aggregate level. However, firm size alone does not explain the surge in breakthrough innovations during this period. I show that the emergence of corporate R&D labs had an independent positive effect on innovation productivity. These findings suggest that organizational structure, not just scale, drove technological progress before World War II.