Several mechanisms of action, both biological and psychological, have been hypothesized to underlie the anti-depressant effects of yoga practices. These effects may encompass acute, transient responses that occur during or immediately after a yoga session, as well as longer-term adaptive responses following yoga practice for several months or more. Biological mechanisms may include [64,82,148–155] down-regulation of the hypothalamic-pituitary-adrenal axis and sympathetic nervous system, increase in vagal tone with activation of the parasympathetic nervous system, increase in central nervous system neurotransmitters such as serotonin and gamma-aminobutyric acid (GABA), increase in brain-derived neurotropic factor (BDNF), promotion of cerebral alpha wave activity, and increase in telomerase activity with mitigation of depression-related cellular decline. Several psychological mechanisms have also been proposed. To the extent that yoga may offer experiences of mastery and self-acceptance, feelings of self-efficacy and self-esteem may increase [102–107] and mediate recovery from depression [108–113]. Mindfulness aspects of yoga may reduce negative ruminations in depression [87,156], leading to reduced feelings of distress [157–159]. Moreover, the decision to learn yoga may facilitate behavioral activation, a psychological mechanism by which lifestyle choices that disrupt depressive routines and lead to increased social connection or other experiences of reward become positively reinforced, thereby mediating additional adaptive behavioral change with further mood benefit [160–162]. Despite plausible support for several hypotheses, much more investigation is required on how yoga may exert mood effects.