Why do some armed groups kidnap for ransom? Despite a dramatic spike in kidnappings by political groups over the last several decades, there are scant existing explanations for why groups use this tool of coercion. Leveraging evidence from extensive interviews with former combatants from Colombia’s civil war, including the Revolutionary Armed Forces of Colombia (FARC) and National Liberation Army (ELN), as well as military and security personnel, I show that ransom kidnapping is used to enforce groups’ protection rackets, their main source of funding. Kidnapping is both the most lucrative way to punish tax evasion and an effective means of deterring future shirking. Thus, groups that tax local populations are more likely to kidnap; groups relying on external or voluntary forms of funding are less likely to take hostages. This article explains when we should see kidnapping in armed conflict, describing an underexplored way that selective violence bolsters insurgency.