This systematic review collates studies of dietary or bariatric surgery interventions for obesity using positron emission tomography and single-photon emission computed tomography. Of 604 publications identified, 22 met inclusion criteria. Twelve studies assessed bariatric surgery (seven gastric bypass, five gastric bypass/sleeve gastrectomy), and ten dietary interventions (six low-calorie diet, three very low-calorie diet, one prolonged fasting). Thirteen studies examined neurotransmitter systems (six used tracers for dopamine DRD2/3 receptors: two each for 11C-raclopride, 18F-fallypride, 123I-IBZM; one for dopamine transporter, 123I-FP-CIT; one used tracer for serotonin 5-HT2A receptor, 18F-altanserin; two used tracers for serotonin transporter, 11C-DASB or 123I-FP-CIT; two used tracer for μ-opioid receptor, 11C-carfentanil; one used tracer for noradrenaline transporter, 11C-MRB); seven studies assessed glucose uptake using 18F-fluorodeoxyglucose; four studies assessed regional cerebral blood flow using 15O-H2O (one study also used arterial spin labeling); and two studies measured fatty acid uptake using 18F-FTHA and one using 11C-palmitate. The review summarizes findings and correlations with clinical outcomes, eating behavior, and mechanistic mediators. The small number of studies using each tracer and intervention, lack of dietary intervention control groups in any surgical studies, heterogeneity in time since intervention and degree of weight loss, and small sample sizes hindered the drawing of robust conclusions across studies.