Fully developed teleosts possess a highly developed immune system comprising both innate and adaptive ele-ments, but when hatching from the egg, the yolksac larva is still at an ontogenetically incomplete stage withregard to physiological, including immunological, functions. The immune system in these young fish stages is farless developed when compared to the youngs appearing from reptile and avian eggs and from most mammals atparturition. In those vertebrate groups the early ontogenetic development of the fetus is highly protected. Thelack of a fully developed immune system in yolksac larvae of fish is critical, because this stage encounters apotentially hostile and infectious aquatic environment. The strong selective pressure on the immune system ofthe yolksac larva and the youngest fry stages explains the existence of a multi-facetted innate system, which isprotecting the young fish stages against viral, bacterial and parasitic infections. The sequential development ofimmune cells and organs depends on host species and its environmental setting. However, a strong armamentcomprising innate cells (neutrophilic granulocytes, macrophages) and molecules (receptors, lectins, complement,AMPs and constitutively expressed immunoglobulins) protect the earliest stages. The adaptive immune elements,including T-cells and B-cells, occur gradually in headkidney, spleen, thymus, tonsils, bursa equivalent (if present)and mucosa associated lymphoid cells. A functional protective response following immunization occur later.