Neurovariability refers to the naturally occurring differences in brain structure and functional organisation across individuals. Rather than being noise, this variability reflects an adaptive feature of human neurobiology, supporting diversity, flexibility, learning, and resilience across evolution and lifespan. Here, we summarise the anatomical, functional, and evolutionary foundations of neurovariability, its relevance to health and disease, and the current limitations of methods for quantifying it. By understanding neurovariability as an emergent property of complex systems, we highlight its importance in advancing personalised neuroscience.