Early life stress is a research concept that investigates the impact of adverse experiences occurring during critical periods of development on subsequent biological, psychological, and social outcomes across the lifespan. This concept encompasses various forms of adversity, including established adverse childhood experiences (ACEs) and emerging factors such as the unpredictability of early environmental and sensory signals. Research within this domain explores the multifaceted mechanisms by which these stressors disrupt typical developmental trajectories, including alterations in neurodevelopment, epigenetic regulation, and physiological system function. The study of early life stress is significant due to its robust association with increased risk for a wide spectrum of negative long-term consequences, including mental health disorders, cognitive deficits, and chronic physical illnesses, underscoring its substantial clinical relevance and economic burden, while also informing potential preventative and therapeutic interventions.