Infant stimulation plays a crucial role in early brain development, influencing cognitive, emotional, and behavioural outcomes across the lifespan. The first 1000 days of life represent a sensitive period characterized by rapid neurodevelopment and heightened plasticity, during which environmental inputs significantly shape neural architecture. This review synthesizes classical developmental theories including maturational, cognitive, behaviourist, attachment, and ecological frameworks with contemporary neuroscience and empirical evidence from PubMed-indexed literature. Emerging research highlights the importance of multisensory stimulation, caregiver interaction, and early intervention programs in optimizing developmental outcomes, particularly among high-risk populations such as preterm infants. Evidence from randomized controlled trials and systematic reviews demonstrates that early stimulation improves neurodevelopmental outcomes, enhances parent–child interaction, and contributes to long-term cognitive and psychosocial functioning. However, gaps remain in culturally contextualized interventions and long-term follow-up studies. This review provides an integrated theoretical framework and emphasizes the need for scalable, evidence-based stimulation programs globally.