Which firms drive aggregate productivity growth? Price-earnings ratios differ markedly across publicly-listed firms.  Large differences remain after netting out proxies for firm-specific discount factors.  We find that high P/E firms tend to see increases in their earnings relative to sales, which we interpret as rents from ideas. We construct an endogenous growth model with persistent shocks to firm innovation step-sizes and calibrate it to match patterns in the data. The model implies that growth would be less than half as fast, even with the same innovative effort, if firms had the same step sizes.  The model can be used to infer expected growth contributions of individual firms (such as members of the Magnificent Seven) and individual sectors (such as AI firms). We find that the share of growth coming from the smallest listed firms exceeds their 10% sales share, whereas the largest firms account for less than their 10% sales share.