Developmental milestones represent the acquisition of functional skills across four domains: gross motor, fine motor/adaptive, language/communication, and social/emotional. The neurobiological basis of development involves progressive myelination of neural pathways (cephalocaudal and proximodistal patterns), synaptic pruning based on environmental stimulation, and critical periods of brain plasticity. Motor development follows predictable patterns: cephalocaudal (head control before trunk before legs) and proximodistal (shoulder control before hand before finger). At the NP level, developmental surveillance involves standardized screening tools (ASQ-3 at 9, 18, 30 months; M-CHAT-R/F at 18 and 24 months for autism), distinguishing normal variants (late walkers, bilingual language delay) from pathological delays, and understanding red flags requiring urgent referral (loss of previously acquired milestones = regression, concerning for neurodegenerative disease or autism). The NP formulates differential diagnoses for developmental delay (genetic syndromes, metabolic disorders, hearing/vision impairment, environmental deprivation, prematurity-related), orders appropriate investigations (hearing screen, thyroid function, lead level, genetic testing, brain MRI), and coordinates early intervention services. Early identification and intervention during critical periods of brain plasticity significantly improve long-term outcomes.