If you’ve been watching the early adopters of TSA Pre-Check whiz through their special short lines and you’re starting to think it might be worth finding out the secret password, here’s a quick primer to Global Entry and TSA Pre-Check.

If you take even one international flight per year, enroll in Global Entry. The program speeds American nationals and residents (and citizens of some other countries) through re-entry to the United States. You skip the lines and scan your passport and fingerprints at the Global Entry kiosks at immigration. You can enroll your children too—not having to chase your two-year old after she makes a break for it past 100 people in line and the immigration official as though claiming sanctuary (not that cute after a 16-hour flight home from Australia) will have you thanking yourself for enrolling ahead of time.

The added bonus is that if you have Global Entry, you’re automatically eligible for TSA Pre-Check on any flight, domestic or international. You do have to contact your airline and make sure your Global Entry Number is registered with them; that way your TSA Pre-Check approval (if available—more on that below) will be on your boarding pass.

Global Entry has a one-time $100 fee and lasts for five years. You enroll by filling out a form online, then you’ll be asked to come for an interview at an Enrollment Center (most of them are airports).

Start your application well in advance of your international flight, as the program is growing in popularity faster than they can open centers to handle all the interview requests and it can take weeks to months to get an interview.

TSA Pre-Check seems to operate according to the principles of random intermittent reinforcement. Like rats in the lab, sometimes we get the sugar pellet: no standing spread-eagled in a futuristic pod with everyone watching you! And sometimes we don’t: yes, spread-eagled, and shoes-off, too. To be fair, the TSA is working so quickly to get more travelers approved and to add Pre-Check application centers that by the time you read this, the situation will likely have changed. But here’s a quick look at the program as it now stands.

You can qualify for pre-check by enrolling via the TSA Pre-Check application program, by being invited by your airline if you’re a frequent flier with a certain status, if you’re a member of the U.S. Armed Forces, if you’re part of a “Trusted Traveler” program like Global Entry, or if you receive a random, not consistent and not frequent (their words, not ours) invitation from the TSA, which will just appear on your boarding pass.

Still with us? Okay then! To apply: enroll online, then go to an enrollment center. There is a one-time fee of $85 and it lasts five years. As with Global Entry, make sure you give your Known Traveler Number to your airline before you fly.

Now that you have Pre-Check as part of Global Entry or on its own, you get to skip the TSA lines. Sometimes. Pre-Check is only available at some airports, through participating airlines, some of the time. You could, for example, be eligible for Pre-Check through Global Entry, have registered with your participating airline and be flying into a participating airport, and still not have Pre-Check available—though the program is rapidly expanding.

In a nutshell:

Global Entry—Easy to apply, easy to understand, includes TSA Pre-Check, and it works.

TSA Pre-Check—Inconsistent and random, $15 cheaper than Global Entry, and it works, sometimes, but you won’t know when or why.

Our advice? Take an international trip once in a while to make Global Entry worth it—here’s some inspiration (we’ll take Barcelona). Speed through immigration on your way home, and if you don’t make the short line at security, keep your cool and win karma points for being nice to the long-suffering TSA agent.

Happy travels!