© The Author(s) 2019. From its origins, human attachment has always been viewed as a primary, biologically-based phenomenon with strong evolutionary roots. The last few decades have seen a large increase in research studies from developmental science and neighbouring disciplines that attest the role of biology in attachment processes. This review aims to provide a concise and up-to-date summary of research on the biological bases of attachment throughout the lifespan. We review the role of genetics, physiology ‒ focusing on oxytocin and cortisol ‒ and brain mechanisms that underlie attachment behaviour and its consequences. Findings are complex in that they often do not apply equally to all patterns of attachment, to all ages, or all temperament styles, among other factors. In addition, some important evidence is indirect, coming from studies that investigate the impact of variations in caregiving quality, such as differences in maltreatment and separation from caregiving figures, and on biology, and vice versa, rather than looking at differences in attachment per se, which complicates their interpretation. Implications for practice are raised throughout.