Contact binaries below the short-period limit of 0.22 days are very rare. Here we define them as the "extremely short-period contact binaries". Observations and investigations of extremely short-period contact binaries can provide important information on the origin and evolutionary processes of contact binaries. We have monitored some such binaries for several years using several small telescopes as well as 2 m-class telescopes. We detected some new extremely short-period contact binaries and derived photometric solutions and period changes for them. Possible reasons for the low occurrence of the extremely short-period contact binaries are discussed.