Using data on over 900 firms for the period 1988-2000, we estimate the effect on phase-specific biotech and pharmaceutical R&D success rates of a firm's overall experience, its experience in the relevant therapeutic category, the diversification of its experience across categories, the industry's experience in the category, and alliances with large and small firms. We find that success probabilities vary substantially across therapeutic categories and are negatively correlated with mean sales by category, which is consistent with a model of dynamic, competitive entry. Returns to experience are statistically significant but economically small for the relatively straightforward phase 1 trials. We find evidence of large, positive and diminishing returns to a firm's overall experience (across all therapeutic categories) for the larger and more complex late-stage trials that focus on a drug's efficacy. There is some evidence that a drug is more likely to complete phase 3 if developed by firms whose experience is focused rather than broad (diseconomies of scope). There is evidence of positive knowledge spillovers across firms for phase 1. However, for phase 2 and phase 3 the estimated effects of industry-wide experience are negative, which may reflect either higher Food and Drug Administration (FDA) approval standards in crowded therapeutic categories or that firms in such categories must pursue more difficult targets. Products developed in an alliance tend to have a higher probability of success, at least for the more complex phase 2 and phase 3 trials, and particularly if the licensee is a large firm.