Myeloid cells are essential in immunity: they control the interaction of the host with the environment and are indispensable in fighting infections and in the communication with other branches of the immune system. The introduction of new technology is revealing unprecedented levels of complexity in ontogeny, specialization, regulation and modulation of adaptive immunity by these cells. This Keystone Symposia meeting will cover topical questions in myeloid cell biology, including new concepts like the origin of myelocytes, trained immunity, and epigenetic and genetic regulation of their function. It will cover myeloid cells of both bone marrow and fetal origin with an emphasis on how macrophages interact with other myeloid cells including granulocytes. There will also be an emphasis on clinical approaches including medical intervention. New approaches to understand the diversity and specificity of myeloid responses will be also included. The symposium will include different fields central to contemporary immunology both at the genetic and cellular level, including the instruction of the adaptive immune response by innate immunity in health and disease. It should therefore be of interest to both basic and clinical scientists working in cell biology, immunology and infectious disease.