At home whitening such as strips and trays do work. However, professional teeth whitening done at a dentist's office is superior for a couple of reasons. First, at home kits with trays use trays that are not custom fit to your mouth. This may cause the whitening solution not to reach all areas of your teeth leaving an uneven look. In addition, since the trays are not custom, they may push too much of the whitening solution onto your gums which can cause a lot of irritation and redness in your gums. When you whitening at a dentist office, they make custom trays to fit your teeth. This eliminates the uneven whitening that may occur with home trays. Also, the trays do not overlap the gums and help with gum irritation. And finally, the dentist is able to use a much stronger whitening solution that you can get for at home use which speeds the process. So most people can get their teeth whitening to their satisfaction in just one visit versus at home treatments which can take a week or longer.