Walk into any local running store and you’ll face a colorful wall holding dozens of shoes. Needless to say, finding the best-fitting shoe among numerous choices isn’t easy. To make matters more complicated, not every shoe is right for you. Whichever pair you choose must fit properly from heel to toe and feel comfortable with your regular running stride.

When considering what running shoes you should buy, remember that most of us prefer to shop online these days. But with that, there’s a little more risk involved if you don’t get to try on before you buy. Luckily, most retailers will offer a risk-free trial period so you can still lace up your shoes and head out for a test run, like you would at the store. (Just double check the return policy, and always keep the box in case you do need to send them back or swap sizes.)

How Your Shoes Should Fit

Each part of the running shoe has a specific purpose and is designed to fit the foot a certain way. Even the slightest differentiation may affect your experience. We’ve broken down the running shoe’s main elements so you can easily identify them and make sure each fits your foot properly when deciding what to buy. (For a deeper dive, you can learn more about the anatomy of a running shoe.)

Everything above the sole counts as part of your running shoe’s upper. Traditionally, shoe brands crafted their uppers with layers of fabrics and mesh sewn and glued together. Today’s modern versions increasingly use knitting and 3D printing to create seamless one-piece fits that stretch and support in appropriate places. It should lay smooth wherever it touches—not binding, chafing, or bunching anywhere.

The ankle collar is the wrap at the top of the shoe opening that holds the heel down in place. Some shoes use thick padding, while others rely more on the shape to cradle the ankle bone. Pay attention to whether your heel slips, how the padding interacts with the bones on the side of your ankles, and whether the curve on the back irritates your Achilles tendon.

The heel counter is a semi-rigid cup layered inside the rearfoot that cradles and supports your heel. Some shoes have an external heel wrap that serves a similar function while more minimalist shoes have eliminated the heel counter to allow full freedom of movement. Research has shown that heel counters do not provide motion control, but they do center the heel for stable landings and support. (Both neutral shoes and stability shoes alike utilize heel counters.) Look for a heel that allows a comfortable ankle motion.

The saddle of the shoe is the reinforced area around the instep, the arch of your foot between the ball and the ankle. A saddle interacts with the laces to hold the shoe securely on the foot. Designers have developed a variety of overlays, eyelets, and lacing systems to mold them closely to any foot shape. Pay attention to how it fits and holds your foot, providing a secure feeling with no slippage while allowing for the natural doming of the arch during your stride.

The toebox encompasses all of the upper from the front of the eyelets to the end of the shoe. It’s often capped with a reinforced toe bumper that holds the fabric off your toes and protects from stubbing, particularly in trail shoes. Look for a wide toebox that stays out of the way, allowing your forefoot to flex and spread out naturally in both width and length. It shouldn’t cramp or rub your toes, either—not even your pinky toe. Ideally, you should be able to wiggle each digit comfortably inside the shoe.

Consider Your Running Surface

The outsole of your running shoe is where the rubber meets the road. It is often made of a variety of rubber or foam compounds placed in strategic areas to increase wear life or enhance bounce or flexibility. Materials that provide traction and durability without adding excess weight or stiffness, and for a footprint shape that matches yours and gives you the desired level of stability underfoot.

Midsole Shape

To make the shoe bend like your foot bends, many shoes use flex grooves under the ball of the foot. Turning the toe up, called toe spring, or cutting away the midsole into a rocker pattern also allows the foot to roll through the stride. Small differences in location or angle can alter the mechanics and feel, and what degree of flex works best for your stride as it changes with speed. Rocker-style midsoles tend to have more aggressive toe springs to facilitate a quick forward roll through the gait cycle. Look for a shoe that flexes or rolls the way your foot wants to move—at the pace for which you’ll be using the shoes.

The midsole is foam material between the outsole and the upper, designed to cushion the runner from impact forces and guide the foot through the stride. Every runner will have their own preferences. Choose a midsole thickness and material that feels right at running speeds, meets your desired level of softness or firmness, and doesn’t have excess weight.

Heel and Forefoot Cushioning

Heel cushioning is the midsole material designed to minimize the impact shock of a heel strike. Besides using a variety of cushioning materials, some shoes feature a softer “crash pad” area on the outer edge of the foot or a rounded outer heel to smooth the landing. Research has shown that the body provides the majority of cushioning for your joints and that you land harder in a more cushioned shoe, so heel cushioning is largely a matter of perceived comfort. You’ll likely want a balance between cushioning, stability, and ground feel. During your test runs, note whether the shoe touches down where you expect it to and rolls into the stride smoothly.

Forefoot cushioning is designed to reduce the impact of the largest forces of the stride that occur at the front of the foot during loading and push off. While body mechanics largely provide cushioning to everything above the ankle, forefoot shoe cushioning protects the structures of the foot. The promise of new “energy-return” materials and designs is that they can both protect and propel your foot. Pay attention to the shoe’s responsiveness, looking for a balance between cushioning comfort and a firm push-off platform.

Heel-Toe Drop

Drop is the difference in height between your heel and the ball of your foot when standing in the shoe. Experts disagree on the importance of drop related to injuries (remember Vibrams?), but agree that changing drop distributes forces differently to the foot and leg, and can alter your stride. Pick a shoe that feels right throughout the stride, from touchdown to toe-off, and reduces stress on any weak parts of your foot. Zero-drop shoes, like those made by Altra, place both the heel and toe equidistant from the ground.

Assess Your Gait and Running Style

Designers use a variety of technologies (such as medial posts, dual-density foams, varus wedges, guide rails, and wider shoe geometries) to try to keep the foot from excessive motion, primarily overpronation or rolling inward. Scientists agree that most people do not need pronation support, but control and stability devices appear to help some runners maintain their preferred movement path. Your shoe should offer stability as support, not over-correction. If you overpronate, you may want a shoe that provides more of these stabilizing features.

Try a New Insole

A sockliner, also called an insole, is the removable pad of foam inside the shoe that cushions the contours of the bottom of your foot. Along with shoe geometry, it provides most of what people think of as “arch support” and gives the shoe its initial step-in comfort. Pay attention to how the shoe feels on the run, where softer is not always better and the foot works dynamically to provide its own support and cushioning. If you wear an orthotic, make sure it also fits inside your shoe comfortably.

Avoid Common Shoe-Buying Mistakes

Specialty running store staffers see runners making the same mistakes again and again when they come in to buy shoes. But not you, not anymore, thanks to this advice from five prominent store owners and managers.