Animal venoms comprise numerous toxin families, consisting mainly of peptides and proteins. In prey, these toxins affect various vitally important systems that may result in severe illness or death. During evolution, toxins acquired the ability to bind selectively and with high affinity to different biological targets in organisms. However, at present, not all toxin targets have been identified, and not all the molecular mechanisms underlying the effects of toxins are understood. This understanding is very important for the efficient treatment of envenomation, which still continues to be a significant problem. On the other hand, the high selectivity and efficiency of toxins make them valuable molecular tools for fundamental research. Moreover, toxins with known mechanisms of action may serve as templates for drug development. All this suggests that studying the molecular mechanisms of action of animal venoms and their toxins is a very challenging and important task. The aim of this Special Issue of Toxins is to present a modern understanding of the various aspects of the molecular mechanisms underlying the action of animal venoms and their constituents.