Make sure a sharpener is actually what you need
As cooks are fond of pointing out, a dull kitchen knife is a dangerous knife, because it's more likely to slip and slice your fingers, instead of whatever else you were working on. Even the most expensive knives can slide into that danger zone as they lose sharpness over time. You can send your knives out to a professional for re-sharpening, but with fewer and fewer sharpening services available locally, doing it yourself is more attractive than ever. That way you don't lose the use of your knives while they're being shipped back and forth, and you also save the money you would have paid to the sharpening service.
The term "sharpening" is often used to describe both honing (re-straightening the burr that forms the cutting edge of a knife) and reshaping the blade to create a new burr. Most in-home knife sharpeners are capable of doing both to some degree. Sharpening your knives before the blade becomes nicked or significantly dull makes the sharpening process easier and faster, and also helps preserve your knives because in order to reshape a dull blade you have to remove a lot of steel. That makes being able to sharpen your knives on the spot -- as opposed to waiting until they're in bad shape before sending them out -- that much more convenient.
Good cutlery makes a difference, too; see our report on kitchen knives for the best knives that will hold an edge longer and sharpen up more quickly. If you find that you're uncomfortable working with an exposed blade or need a slicer that quickly makes many uniform cuts, we've also evaluated the best mandolines to make short work of any pile of veggies.
Home sharpeners use some sort of an abrasive -- either tungsten carbide, ceramic, steel or diamond, which provides the hardest, most aggressive sharpening surface, to reshape the knife blade. Most have at least two sharpening surfaces to choose from; you start with a coarser grit to remove more steel, then use a finer grit to polish your knife to a smooth edge.
Do you need a manual or electric knife sharpener?
Whether manual or electric, each type of sharpener comes with its own set of trade-offs. Manual knife sharpeners are generally more compact and affordable. In fact, we found excellent reviews for one small manual sharpener that costs less than $10. Fancier manual models, which may cost $50 or more, offer users more control over the sharpening process but also require more skill to use. Most manual sharpeners have no moving parts at all; you do the work by drawing the blade repeatedly across the sharpener's abrasive surface.
Electric knife sharpeners don't provide much control, but they're faster and easier to use. They also tend to be larger -- nearly the size of a toaster -- and are designed to sit on your countertop. In almost every case, you pull the knife slowly through specially designed slots in the sharpener; abrasives hidden inside the slots do the sharpening. Expect to spend around $150 for the convenience of an electric sharpener.
The angle at which you position the knife during sharpening is key. Most electric sharpeners have guides to help you maintain the blade at the right angle, and some can accommodate multiple blade angles. European/Western knives typically have a blade angle of 20 degrees, while Asian knives typically have a blade angle of 15 degrees, although many European knife makers have started introducing 15-degree blades as well.
Manual blade sharpeners come in a wider variety of styles. One of the most common styles is a rod-style sharpener like our best-reviewed Spyderco 204MF Tri-Angle Sharpmaker System (Est. $60), which holds two ceramic rods in the base at preset angles. You pull the knife in strokes against first one rod, then the other. This does take a little practice to master but, once you have it, the process is quick and effective.
Sharpening/honing rods are another common type of knife sharpener, although none qualified for our best-reviewed picks in this report, mainly because they're great for honing a good edge but not for reshaping a truly dull edge. Honing rods work much like the rods in the sharpeners mentioned above but, since there's no base, you hold the rod steady with one hand while pulling the knife across it with your other hand.
Slotted manual sharpening systems are simpler; you simply pull the knife repeatedly through a slot that's designed to hold it at the proper angle, much as you would with an electric sharpener. However, the process takes a little longer because, unlike an electric sharpener, the abrasive surfaces are not motor-driven.
The trickiest sharpeners to use are flat whetstones, such as Japanese water stones. Although many experts like the control whetstones offer, they admit that it takes a good bit of practice to master them—and using them incorrectly can ruin your knife's blade. We did not find any sharpeners of this type that received strong recommendations from home users.
How we found the best knife sharpeners
To select our best-reviewed knife sharpeners, we considered four factors, performance, ease of use, safety and, for electric sharpeners, the noise level. To rate knife sharpeners on these criteria, we consulted professional comparison tests at sites such as Cook's Illustrated and Wired.com. We also looked at more casual tests in newspaper articles and on consumer websites. Finally, we consulted hundreds of reviews at retail sites such as Amazon.com to see how knife sharpeners perform in the hands of typical home users. The result is our picks for the best knife sharpeners for any budget and level of experience.