In situ gamma spectroscopy was introduced to determine the outdoor gamma dose rate from soil and to calculate the radionuclide concentration and the relative contribution to the dose rate. This paper reviews the most common and proven applications of in situ gamma spectroscopy-together with the most recent and innovative research outcomes obtained with this technique, particularly for its use indoors. Advantages and limitations of its utilization to assess environmental radioactivity-indoors and outdoors-are also discussed.