Intervention at the earliest illness stage, in ultra or clinical high-risk individuals, or indicated prevention, currently represents the most promising strategy to ameliorate, delay or prevent psychosis. We review the current state of evidence and conduct a broad-spectrum meta-analysis of various outcomes: transition to psychosis, attenuated positive and negative psychotic symptoms, mania, depression, anxiety, general psychopathology, symptom-related distress, functioning, quality of life, and treatment acceptability. 26 randomized controlled trials were included. Meta-analytically pooled interventions reduced transition rate (risk ratio [RR] = 0.57, 95%CI 0.41-0.81) and attenuated positive psychotic symptoms at 12-months (standardized mean difference = -0.15, 95%CI = -0.28--0.01). When stratified by intervention type (pharmacological, psychological), only the pooled effect of psychological interventions on transition rate was significant. Cognitive behavioral therapy (CBT) was associated with a reduction in incidence at 12-months (RR = 0.52, 95%CI = 0.33-0.82) and 18-48-months (RR = 0.60, 95%CI = 0.42-0.84), but not 6-months. Findings at 12-months and 18-48-months were robust in sensitivity and subgroup analyses. All other outcomes were non-significant. To date, effects of trialed treatments are specific to transition and, a lesser extent, attenuated positive symptoms, highlighting the future need to target other symptom domains and functional outcomes. Sound evidence supports CBT in reducing transition and the value of intervening at this illness stage. STUDY REGISTRATION: Research Registry ID: reviewregistry907.