The concept of the neonatal window of opportunity assigns the early postnatal period a critical role for life-long host-microbial and immune homeostasis. It is supported by epidemiological evidence that links postnatal environmental exposure with disease susceptibility and mechanisms in the neonate host that facilitate the postnatal transposition, establish a stable microbiome and promote immune maturation. During the conference on "The neonatal window of opportunity - early priming for life", postnatal microbiome and immune maturation, epidemiological evidence and fundamental mechanisms were discussed to identify new targets for future preventive and interventional measures.