The colony stimulating factors (CSFs), granulocyte macrophage-CSF (GM-CSF), macrophage-CSF (M-CSF or CSF-1) and granulocyte-CSF (G-CSF) were first identified as in vitro hematopoietic growth factors. They have since been shown to regulate myeloid cell numbers and function at steady state and during inflammation. Preclinical data suggest that targeting CSFs might be beneficial in autoimmune and inflammatory disease, and manipulation of CSF biology is now being tested in clinical trials. Here, we examine recent insights into CSF function, at steady state and during pathology, as provided by CSF or CSF receptor neutralization/deletion studies or from CSF administration. We discuss controversies regarding the role of CSFs in controlling specific myeloid cell populations and highlight how the newly identified M-CSF receptor ligand, interleukin (IL)-34, is necessitating a reassessment of the field.