Background: The term 'medically unexplained symptoms' is used to cover a wide range of persistent bodily complaints for which adequate examination and appropriate investigations do not reveal sufficiently explanatory structural or other specified pathologies. A wide range of interventions may be delivered to patients presenting with medically unexplained symptoms in primary care. Many of these therapies aim to change the behaviours of the individual who may have worsening symptoms. Objectives: An evidence synthesis to determine the clinical effectiveness and cost-effectiveness of behavioural modification interventions for medically unexplained symptoms delivered in primary care settings was undertaken. Barriers to and facilitators of the effectiveness and acceptability of these interventions from the perspective of patients and service providers were evaluated through qualitative review and realist synthesis. Results: Fifty-nine studies involving 9077 patients were included in the clinical effectiveness review. There was a large degree of heterogeneity both between and within intervention types, and the networks were sparse across all outcomes. At the end of treatment, behavioural interventions showed some beneficial effects when compared with usual care, in particular for improvement of specific physical symptoms [(1) pain: high-intensity cognitive-behavioural therapy (CBTHI) standardised mean difference (SMD) 0.54 [95% credible interval (CrI) 0.28 to 0.84], multimodal SMD 0.52 (95% CrI 0.19 to 0.89); and (2) fatigue: low-intensity cognitive-behavioural therapy (CBTLI) SMD 0.72 (95% CrI 0.27 to 1.21), relaxation/stretching/social support/emotional support SMD 0.87 (95% CrI 0.20 to 1.55), graded activity SMD 0.51 (95% CrI 0.14 to 0.93), multimodal SMD 0.52 (95% CrI 0.14 to 0.92)] and psychological outcomes [(1) anxiety CBTHI SMD 0.52 (95% CrI 0.06 to 0.96); (2) depression CBTHI SMD 0.80 (95% CrI 0.26 to 1.38); and (3) emotional distress other psychotherapy SMD 0.58 (95% CrI 0.05 to 1.13), relaxation/stretching/social support/emotional support SMD 0.66 (95% CrI 0.18 to 1.28) and sport/exercise SMD 0.49 (95% CrI 0.03 to 1.01)]. At short-term follow-up, behavioural interventions showed some beneficial effects for specific physical symptoms [(1) pain: CBTHI SMD 0.73 (95% CrI 0.10 to 1.39); (2) fatigue: CBTLI SMD 0.62 (95% CrI 0.11 to 1.14), relaxation/stretching/social support/emotional support SMD 0.51 (95% CrI 0.06 to 1.00)] and psychological outcomes [(1) anxiety: CBTHI SMD 0.74 (95% CrI 0.14 to 1.34); (2) depression: CBTHI SMD 0.93 (95% CrI 0.37 to 1.52); and (3) emotional distress: relaxation/stretching/social support/emotional support SMD 0.82 (95% CrI 0.02 to 1.65), multimodal SMD 0.43 (95% CrI 0.04 to 0.91)]. For physical functioning, only multimodal therapy showed beneficial effects: end-of-treatment SMD 0.33 (95% CrI 0.09 to 0.59); and short-term follow-up SMD 0.78 (95% CrI 0.23 to 1.40). For impact on daily activities, CBTHI was the only behavioural intervention to show beneficial effects [end-of-treatment SMD 1.30 (95% CrI 0.59 to 2.00); and short-term follow-up SMD 2.25 (95% CrI 1.34 to 3.16)]. Few effects remained at long-term follow-up. General practitioner interventions showed no significant beneficial effects for any outcome. No intervention group showed conclusive beneficial effects for measures of symptom load (somatisation). A large degree of heterogeneity was found across individual studies in the assessment of cost-effectiveness. Several studies suggested that the interventions produce fewer quality-adjusted life-years than usual care. For those interventions that generated quality-adjusted life-year gains, the mid-point incremental cost-effectiveness ratios (ICERs) ranged from £1397 to £129,267, but, where the mid-point ICER fell below £30,000, the exploratory assessment of uncertainty suggested that it may be above £30,000. Conclusions: Behavioural interventions showed some beneficial effects for specific medically unexplained symptoms, but no one behavioural intervention was effective across all medically unexplained symptoms. There was little evidence that these interventions are effective for measures of symptom load (somatisation). General practitioner-led interventions were not shown to be effective. Considerable heterogeneity in interventions, populations and sparse networks mean that results should be interpreted with caution. The relationship between patient and service provider is perceived to play a key role in facilitating a successful intervention. Future research should focus on testing the therapeutic effects of the general practitioner-patient relationship within trials of behavioural interventions, and explaining the observed between-study differences in effects within the same intervention type (e.g. with more detailed reporting of defined mechanisms of the interventions under study).