The global proliferation of quotas for women over the past 30 years is both remarkable and consequential. Targeting decision-making positions historically resistant to women's equal inclusion, the adoption of electoral and corporate board quotas has at times been controversial. After adoption, quotas have influenced women's numbers, the performance and outcomes of decision-making bodies, and broader public attitudes. In this review, we distinguish among types of electoral and corporate quotas, trace arguments for and against the adoption of quotas, and review research on factors that influence quota adoption across time and space. After outlining the methodological difficulties in demonstrating an impact of gender quotas, we review research that is able to isolate an impact of quotas in politics and business. We conclude by providing several suggestions to ensure that future research continues to advance our understanding of the form, spread, and impact of gender quotas.