There is controversy about a causal role of serotonin (5-HT) in depression, some arguing that there is no proof for impaired brain 5-HT function in depressed patients. Major depressive disorder comes with multiple endophenotypes; not surprisingly classical antidepressants (tricyclics, MAO inhibitors, SSRIs, SNRIs) are not universally effective. Most antidepressants target the 5-HT system, partially if not exclusively, but treatment-resistant depression (TRD) remains a major issue. The most recent and heavily investigated class of potential rapid acting antidepressant, anxiolytic, and/or anti PTSD drugs, namely psychedelics (psilocybin, LSD, DMT, ayahuasca, etc..) or entactogens (MDMA, ibogaine), all target the 5-HT system, at least in part. Phase II / III clinical trials support psychedelics- and/or MDMA-assisted psychotherapy as a new class of rapid acting treatments for GAD, MDD, TRD, PTSD, and other disorders. Psilocybin and MDMA have FDA breakthrough status for TRD/MDD and PTSD, respectively, whereas LSD just received FDA breakthrough status for GAD. All psychedelics act as 5-HT2A receptor agonists, although LSD, DMT, psilocybin may also target other 5-HT and/or dopamine receptors. Psychedelics produce rapid onset and long-lasting antidepressant effects after one or two administrations. They all promote synaptogenesis and synaptic plasticity. Neuroinflammation plays a major role in anxiety, depression, PTSD. Interestingly, psychedelic-induced 5-HT2A receptor agonism has profound anti-(neuro)inflammatory effects. Altogether, the 5-HT system plays an essential, but not unique role in MDD and related disorders. MDD, TRD and PTSD may be considered as biochemical, neurological and immune conditions, given the emerging role of neuroplasticity and neuroinflammation, which until recently, have been overlooked.