After a cessation of almost 40 years, there is renewed interest into therapeutic applicationsof the serotonergic psychedelic psilocybin for the treatment of patients with various psychiatric disorders. PubMed was searched for clinical trials into "psilocybin" between 2000 and 2020, complemented by handsearching. Articles were also screened for explanatory models and working mechanisms. Psilocybin has been studied in 9 clinical trials: for the treatment of substance use disorders, depression, end-of-life anxiety, demoralization, and obsessive-compulsive disorder. Results show that psilocybin is well tolerated, with only limited side-effects, while even patients with treatment-resistant disorders sometimes show marked, long-term improvements after one or a few sessions. Initial results are encouraging, but there are several limitations. More research is needed to determine which patient populations can benefit, what role setting and the placebo response play, and how these novel treatments can be optimized.