Yes, you WILL need one. They check for them when you return, but you need to BUY it before you leave INSIDE the airport before inspections. You can only buy it in Pesos, no dollars. They require your passport and you’ll fill in a small card by hand. It’s good for 6 months but you’re expected to hand it in once you leave the COUNTRY (not leave the airport). The cost is about $25 and if you flew from the USA for the extraordinarily higher price, you’d have this part included. If you’re caught without it when you return, they normally just give you a “talking to” however, you could be fined upwards of about $1500 and required to pay before the “deport you” back to the USA. Get one, they go for a good cause. Also it is for tourism only, not for lucrative activities (meaning if your going for business, you’ll need an FM2 or FM3). This website is not about that, so Google it. =)