To evaluate which early neurocognitive and behavioral precursors are associated with the development of attention-deficit/hyperactivity disorder (ADHD) and whether these are currently targeted in early interventions. We conducted 2 systematic reviews and meta-analyses of empirical studies to examine the following: (1) early-life (0-5 years) neurocognitive and behavioral precursors associated with familial likelihood for ADHD, an early ADHD diagnosis/elevated ADHD symptoms, and/or the presence of later-childhood ADHD; and (2) interventions delivered to children aged 0 to 5 years targeting the identified precursors or measuring these as outcomes. Standardized mean differences (Hedges' g) and pre-post-treatment change scores (SMD) were computed. A total of 149 studies (165,095 participants) investigating 8 neurocognitive and behavioral domains met inclusion criteria for part 1. Multi-level random-effects meta-analyses on 136 studies revealed significant associations between ADHD and poorer cognitive (g = -0.46 95% CIs: -0.59, -0.33), motor (g = -0.35 CIs: -0.48, -0.21) and language (g = -0.43 CIs: -0.66, -0.19) development, social (g = 0.23 CIs: 0.03, 0.43) and emotional (g = 0.46 CIs: 0.33, 0.58) difficulties, early regulatory (g = 0.30 CIs: 0.18, 0.43) and sleep (g = 0.29 CIs: 0.14, 0.44) problems, sensory atypicalities (g = 0.52 CIs: 0.16, 0.88), elevated activity levels (g = 0.54 CIs: 0.37, 0.72), and executive function difficulties (g = 0.34 CIs: 0.05, 0.64 to -0.87 CIs: -1.35, -0.40). A total of 32 trials (28 randomized, 4 nonrandomized, 3,848 participants) testing early interventions that targeted the identified precursors met inclusion criteria for part 2. Multi-level random-effects meta-analyses on 22 studies revealed significant intervention-related improvements in ADHD symptoms (SMD = 0.43 CIs: 0.22, 0.64) and working memory (SMD = 0.37 CIs: 0.06, 0.69). Children aged 0 to 5 years with current or later-emerging ADHD are likely to experience difficulties in multiple neurocognitive/behavioral functions. Early interventions show some effectiveness in reducing ADHD symptoms, but their effects on neurocognitive/behavioral difficulties require further study.