While no technology will ever replace diet and exercise, a new class of gadgets may provide key insights into your physical fitness. Known by a litany of terms—fitness trackers, health monitors, activity trackers, and wearables—there’s a growing cadre of tech toys that share a common goal: to get you into better shape.

To know whether any of the trackers are worth your cash, let’s start with what fitness trackers do. Essentially, they are little gadgets users wear for 24 hours a day, seven days a week. The most basic ones are equipped with an accelerometer, which measures motion to collect data activity, steps and sleep.

You can track this data in easy-to-understand graphics online or on a smartphone to literally get a picture of your health. In general, they encourage change with a simple principle: you can only change habits you’re aware of.  As a consumer, you need to figure out exactly what data you want, and how much you’re willing to pay.

So, what should you look for in a tracker?

The best fitness trackers have a built-in altimeter, which measures elevation. A tracker with just an accelerometer might think you’re just walking slowly when you’re climbing 20 flights of stairs and your heart is actually pounding. A tracker with an altimeter will give you a far more accurate picture of calories-burned.

It’s also important to look at form-factor. As in, how do you wear it? Generally, there are two categories: trackers that go in your pocket, and trackers you wear on your wrist. Each has pros and cons.

The pocketable trackers are great because they’re generally very light, small, and completely inconspicuous. Essentially no one would know that you’re wearing one, and so they can be worn at all times, without having to be self-conscious about it (they can typically clip to a bra, too, should you be pocketless). However, there is a major drawback: You have to remember it. That means when you change out of your work-clothes and into your workout gear, you need to remember to take it out of the one pocket and put it in the other. Then move it back again into you post-workout clothes. And if you want to monitor your sleep patterns (which many of these do), you need to pull it out and transfer it to a soft sleeve that goes on your wrist. They are extremely easy to forget and just as easy to lose.

With a wrist-worn tracker, you don’t need to worry about forgetting it in your other pants, or moving it to some weird sleeve thingy when you sleep. The drawback? You’ve always got this thing on your wrist. The thicker, clunkier ones can make typing rather uncomfortable. And, none of the options are particularly fashionable.

The other most important thing is how you get at the information that’s on the tracker. There are two key elements here. For one, better fitness trackers have a screen. It won’t give you the most granular info, but the built-in screen can at least tell you how many steps you’ve taken, floors you've climbed, and calories you’ve burned (estimated), so you know how you’re doing in real time. Secondly, better trackers have a Bluetooth radio, which allows your tracker to sync wirelessly with your computer or, even better, your smartphone. This enables you to get more detail in real time. An app can even alert you when you’re getting close to a goal, or if you’ve been idle for too long. Almost all of the better trackers will sync with the iPhone, and many will sync with newer Android phones, too, but because there is so much variation between Android phones you should always check compatibility before you purchase.

I’ve personally tested just about every tracker that’s come out in the last year and a half, and I currently have three favorites:

The Fitbit Force is terrific. It’s a wrist-worn tracker that has a built-in screen, altimeter, Bluetooth, and it has a long (10 day) battery-life between charges, Plus, it’s quite comfortable and is actually good-looking. You can wear it in the shower no problem (though you shouldn’t swim with it), and Fitbit’s apps are well laid-out. It’s $130.

The Withings Pulse is a truly tiny, pocketable tracker that really packs in the features. It has the same list of accolades as the Fitbit above, but it adds in another neat trick: it can take your pulse. It has to be done manually, by placing your finger over a sensor, but it’s still nice data to have, and again, it’s incredibly inconspicuous. The only drawback is that you have to transfer it to one of those sleeves while you’re sleeping, and you have to try not to lose it in your gym shorts. It gives you a lot for $100.

The Basis B1 is kind of the odd man out here. It looks more like a watch than anything else, and indeed it is, but it also has more sensors than any other tracker out there. Aside from the accelerometer, it has the ability to constantly monitor your heart-rate for several days on end! That means it almost certainly gives you the best information about calorie burn of any tracker. It can also measure how much you sweat and the fluctuations in your body temperature. It’s really smart. Unfortunately, it’s pretty unattractive, a bit buggy, and for something that’s the size of a watch, it lacks some important watch-like features (like timers and such). It also has a battery-life of just a few days (as opposed to ten days for the two above), it has no altimeter, and it’s pricier at $200. That said, if you want as much data as possible, it’s the way to go.