Technologies such as social media, autonomous vehicles, and “big data” analytics generate enormous benefits for society, but they also create substantial harms. Many of these effects take the form of externalities—external benefits and harms that a decisionmaker (such as an inventor) imposes on third parties without charge or compensation. Considering negative externalities, for example, social media networks spread misinformation throughout the electorate, autonomous vehicles threaten the jobs of millions of professional drivers, and predictive policing based on big data can lead to unreasonable searches and seizures. Externalities can cause inefficient resource allocation, and the classic remedy is to “internalize” externalities by ensuring that decisionmakers consider the external benefits and costs of their actions. Patents, which confer exclusive rights on new inventions, enable inventors to internalize a share of the positive externalities from technology, thus shoring up incentives to invent. However, inventions also produce harms, and how patents treat negative externalities from new technologies has been largely overlooked. This Article is the first to extensively examine this issue. It argues that while patents internalize positive externalities associated with innovation, they do surprisingly little to internalize negative externalities. This Article refers to this underappreciated dynamic as patent law’s externality asymmetry.