Gamma scintigraphy has established itself as a method of choice for the in vivo monitoring of the fate of dosage forms. This technique relies on the use of radioactive tracers included into the medicament and selected so as to enable an optimum detection by a gamma ray camera. After a reminder of the basic principles governing the emission of radioactivity and of the safety measures which need to be taken against radioactivity, we shall review the main technological advances which have enabled medical imaging to reach the level of sophistication it now enjoys. A description is also made of the various nuclides used for in vivo evaluation of dosage forms.