Approximately 16.7% of output in high‐income OECD (Organisation for Economic Cooperation and Development) countries is produced informally. I present a model economy where entrepreneurs decide how much of their production to keep informal. Informality carries a risk of getting caught, taxed, and fined. Results from a model with differences in tax rates alone only agree to approximately 23% with data on informality. Taking into account both governance quality and tax rates, agreement between the model's results and data increases to 72%. A policy experiment raising governance quality in Greece, Italy, Spain, and Portugal to Finnish standards reduces informality by 13 percentage points. (JEL E26, H26, J24)