Antimicrobial peptides (AMPs) of invertebrates are characterized by a wide variety of mechanisms of their action that involve not only disruption of the barrier function of a membrane of a target cell, but specific inhibition of metabolism processes via ligand-receptor interactions with specific molecules on the surface or within the cell as well. Endogenous AMPs can play a role of mediators of the immune system (immunomodulators) by activation of phagocytosis and chemotaxis, and by stimulation of production of cytokines. The second part of the review focuses on biological functions and mechanisms of action of AMPs of invertebrates. A problem of biological significance of antimicrobial properties in vitro is reviewed. The main mechanisms of the AMP action on a membrane (models of the barrel-stave channel and the toroidal pore and the “carpet” mechanism) are described, and the problems of selectivity of interaction with a microbial membrane are analyzed. The data on alternative mechanisms of the antimicrobial action, such as inhibition of transcription and translation, sequestration of metal ions, and inhibition of biosynthesis of bacterial and fungal cell walls, are presented. A number of examples demonstrate the AMP regulatory activities of invertebrates.