The design and performance of a high pressure, high temperature scanning tunneling microscope (HPHT-STM) system is described. The system combines an ultrahigh vacuum surface analysis/preparation chamber with a variable pressure (5 x 10(-10) Torr-1 atm) and temperature (300-675 K) STM. The STM chamber can be isolated by three gate valves and filled with 1 arm of any gas mixture. The composition of the gas can be monitored by gas chromatography. A load-lock mechanism allows the transfer of samples and tips into the chamber without exposing it to air. Heating at high pressure is achieved using a halogen lamp beneath the sample. By switching between tungsten and gold tips, the microscope can be used in both oxidizing and reducing environments at room temperature, and reducing environments at elevated temperatures. This instrument allows the exploration of surfaces in a pressure regime that few other surface sensitive techniques can work in.