You can take legal action against him to enforce the terms of any settlement agreement or divorce decree. If he doesn't pay voluntarily and you take him to court and (presumably) win, there are various mechanisms you can sue to recover the money from him. Probably the most commonly used in a case like this would executing or levying on his bank account (also called garnishing his bank account)--taking money directly out from his bank account--or garnishing his wages, which is getting a portion of his income sent directly to you. Note that you can generally garnish more for spousal or child support than you can for other debts (e.g. unpaid loans or credit cards), so garnishing wages is a powerful tool for collecting in cases like this. You should meet with a divorce or matrimonial attorny to discuss your options.