The immune system is crucial for every organism's defence against pathogenic organisms, injuries and abnormal cellular processes.. Cnidarians are a very diverse phylum of animals without developed adaptive immunity, but rely on innate immunity and have developed complex and very effective defense and protection mechanisms against harmful microorganisms. The first line of defence present pattern recognition receptors (PRR) essential for immune recognition, and the best known are TLR receptors, NOD receptors and lectins. These proteins activate the immune signaling modules such as the TLR pathway, the melanin synthesis pathway and complement pathway, which are crucial in processes such as pathogen encapsulation, lysis and wound healing. The endpoints of these signaling cascades are immune effectors, molecules and proteins whose main task is to destroy pathogens. There are several effector responses in cnidarians, and they include the mucosal surface layer, antimicrobial peptides, reactive oxygen species and circulating population of amoebocytes. An important component of cnidarian immunity is the epithelium, which serves as an immune barrier. The epithelium is a habitat for microbial communities and symbionts that are considered important part od the immune system and are essential for the health of an individual organism. This thesis describes in more detail the immunity of cnidarians, including the complex reactions initiated upon the contact with the pathogen and the way in which intruders are isolated and destroyed.