The best type of thermometer — or the best place to insert the thermometer, in some cases — depends on your child's age.

Birth to 3 months. Use a regular digital thermometer to take a rectal temperature. New research suggests that a temporal artery thermometer might also provide accurate readings in newborns.
3 months to 4 years. In this age range you can use a digital thermometer to take a rectal or an armpit temperature or you can use a temporal artery thermometer. However, wait until your baby is at least 6 months old to use a digital ear thermometer. If you use another type of thermometer to take a young child's temperature and you're in doubt about the results, take a rectal temperature.
4 years and older. By age 4, most kids can hold a digital thermometer under the tongue for the short time it takes to get an oral temperature reading. You can also use a digital thermometer to take an armpit temperature, or use a temporal artery thermometer or a digital ear thermometer.