Cnidaria are venomous aquatic organisms, whose dangerousness is remarkable among marine species. Cnidarian jellyfish sometimes show extensive proliferations – outbreaks or blooms – along with occasional strandings on beaches. Due to the invisibility of several diaphanous and completely transparent species, sometimes they are not seen by bathers or sea-workers, resulting in envenomations with consequent local or systemic implications such as contact dermatitis, neurotoxicity, cardiotoxicity and, in serious cases, anaphylaxis. Researchers have historically focused their efforts on the identification of stings, evaluating the shape of lesions, the marks left on the skin, and the etiology of illness, in order to find methods useful to inactivate venoms and to develop care systems aimed at the mitigation of pain, swelling, redness, and systemic consequences. In the interest of better knowing the toxic mechanisms consequent to cnidarian stinging, beginning in the 1990s, research focused on studying the effects of venoms at a cellular level. Since then, a considerable number of papers have provided data about the mechanisms of action of venoms and the associated impacts on cell survival, growth, and metabolism. Subsequently, the utilization of compounds present in jellyfish venoms, both for therapeutical purposes and to counter cell proliferation, has been considered. This paper aims to review the current knowledge about the activity of jellyfish venoms at the cellular level, with a double prospect: i) to implement alternative methods to study cnidarian venoms and ii) to evaluate current opportunities for the therapeutical use of cnidarian extracts.