Magnetic Marker Monitoring offers an alternative to investigate the behavior of solid dosage forms in the organs of the gastrointestinal tract without the need to apply radiation. For Magnetic Marker Monitoring, the dosage form is marked as a permanent magnetic dipole by the incorporation of small amounts of ferromagnetic material, as for example the colorant black iron oxide, and subsequent magnetization. Thereby, the dosage form is labeled as the source of a well defined magnetic field, which can be measured using a measurement technique that is established for biomagnetic investigations. Using the established concepts for magnetic source localization, the three dimensional localization and orientation as well as the strength of the magnetic source can be reconstructed from these magnetic measurement data as a function of time. Furthermore, it is possible to gain quantitative information on the disintegration of dosage forms in vivo. Examples are given for results obtained concerning the esophageal transit, the gastric and the intestinal behavior of capsules and tablets.