There are two main types of teeth whitening treatment: in-office treatment and at-home treatment. In-office treatment involves the use of a high-concentration whitening gel that is applied to the teeth by a dental professional. The treatment usually takes about an hour and can produce immediate results. At-home treatment involves the use of a lower-concentration whitening gel that is applied to the teeth using custom-made trays that are provided by the dental professional. The treatment usually takes several weeks and produces gradual results.