The colony-stimulating factors (CSFs) are a group of cytokines central to the hematopoiesis of blood cells, the modulation of their functional responses, as well as the maintenance of homeostasis and overall immune competence. This group consists of the macrophage-CSF (M-CSF), granulocyte-CSF (G-CSF), granulocyte/macrophage-CSF (GM-CSF), and multi-CSF (IL-3). M-CSF and G-CSF are relatively lineage-specific, having a role in the proliferation, differentiation, and survival of macrophages, neutrophils, and their precursors. In contrast, GM-CSF and multi-CSF function at earlier stages of lineage commitment regulating the expansion and maturation of primitive hematopoietic progenitors. Colony stimulating factor production and degradation are strictly controlled, thus allowing for effective modulation of their biological functions in steady-state conditions as well as under periods of stress. Moreover, the mechanisms behind their expression and that of their cognate receptors ensures that their actions are tightly coordinated, within the context of a network of complex but finely tuned regulatory pathways derived from a variety of local and endocrine hematopoietic regulators. In this review we present some of the most salient information on CSF biology collected over the last three decades. We examine the gene and protein structure of each of the four CSFs and their corresponding receptors, and consider the main determinants behind their biological activities. The components responsible for their functional redundancy as well as the mechanisms that mediate their specificity are also discussed. Although most of available knowledge about CSFs is on human and mouse CSFs, an attempt was made to integrate recent findings in other systems in order to highlight a more widespread role for CSFs throughout evolution.