Quest Diagnostics Incorporated ( ) is a United States corporation which provides clinical laboratory services. The company has operations in the United Kingdom and Mexico, as well as service locations located in Brazil in addition to collaborative agreements internationally with various hospitals and clinics. Quest Diagnostics also has operations in the US Territory of Puerto Rico.

Quest Diagnostics is a member of the Fortune 500 and the S&P 500. The company's corporate headquarters were previously located in Teterboro, New Jersey where the largest business unit is located. Currently, the corporate headquarters are located in Madison, New Jersey having moved there in October 2007 from Lyndhurst, New Jersey.