I used to use the old school whitening strips, but my teeth have been rather dull, so I noticed how much has changed with the Crest whitening strips. Instead of just one kind, there are now several kinds of strips. There are the express strips that you leave on for a short amount of time, the kind for sensitive teeth, and there is glamorous or professional effects. I bought the professional effects one on Amazon because it is sometimes cheaper than buying at the store. It comes with 20 treatments and is in a nice, holographic box. When I use whitening strips, I use the strips everyday until the treatment is up. I try to do this at least once a year. The strips come in pairs and are like stickers. There is a bottom strip and a top strip. They will not whiten the molars, but will whiten whatever teeth are visible. The strips that Crest make nowadays adhere extremely well to the teeth. I just take the strip, make sure it covers the teeth fully and NOT THE GUMS, and then fold over the excess. I whiten before I brush my teeth at night. The reason why is because it adheres better when teeth have plaque and are a little gritty. When they have been brushed, the teeth might be too slimy. I leave it on for thirty minutes or even more. My teeth are not sensitive, but after a week, I might feel pain if I eat cold foods or if I suck in air. This sensation goes away quickly and is a fleeting pain that is so quick that I will just carry on with treatment. After my treatment, my teeth are noticeably whiter and smiling is more enjoyable. Overall, I highly recommend these because they are affordable, readily available anywhere, and are effective!