In infants less than three month of age, rectal temperatures are essential because they provide the most accurate information about core body temperature.  In children older than three months, rectal is still more accurate but many parents prefer to use oral or ear thermometers.  Oral is preferred over axillary (under the arm).  Ear thermometers are usually acceptable; however, they may be inaccurate if there is too much wax in the canal.  Always use a digital thermometer and avoid mercury thermometers.