A decreasing number of new therapeutic drugs reaching the clinic has led to the publication of regulatory guidelines on human microdosing trials by the European Medicines Agency in 2004 and the U.S. Food and Drug Administration in 2006. Microdosing trials are defined by the administration of 1/100(th) of the therapeutic dose and designed to investigate basic drug properties. This review investigates the current application of phase 0 trials in medical research. Thirty-three studies found in PubMed and EMBASE were systematically reviewed for aim and analytical method. Pharmacokinetic studies have been a major focus of phase 0 trials, but drug distribution, drug-drug interactions, imaging, and pharmacogenomics have also been investigated. Common analytical methods were tandem mass liquid chromatography, accelerator mass spectrometry, and positron emission tomography. New ongoing trials are investigating the pharmacodynamics and chemoresistance of marketed drugs, suggesting that the application of phase 0 trials is still evolving.