I've used tres white in the past and this is what my dentist recommends to me unless I want to sit in a chair for $500.00 to whiten my teeth. The gel is preloaded in strips, which sits in a tray for both the upper and lower teeth. Simply stick the trays in and when you are sure the strips are in place, remove the trays. As long as the trays are at room temperature, the strips stay in place and not once have I had a problem with gel oozing out of my mouth. These can stay in for 30-60 minutes each day for ten days. I have sensitive teeth and experienced stinging only on the second day. After that, there was not pain. Something else to note is that my dentist told me to NOT drink coffee, tea, red wine, or soda (like Coke) while whitening my teeth. Your teeth will absorb the color of these products and restain. I noticed a difference in my teeth after the second day. They were definitely whiter. This product is easy to use and definitely whitens your teeth!