Most antimicrobial peptides and their mimics act by permeabilizing the cell membrane of the target organism. A variety of specific modes of action has been proposed and investigated more closely using model membranes. The talk describes a number of classic and recent approaches to study the interactions of membrane-permeabilizing compounds with lipid vesicles. This includes time-resolved fluorescence spectroscopy, for example to quantify and characterize the release of an entrapped dye from the vesicle, microcalorimetry, and zeta potential measurements. Different principal profiles are observed and assigned to different modes of biological activity. It appears that some mechanisms apply to vesicles and cells, whereas other show a poor correlation. These insights are important for the rational optimization of membrane-active agents for medical use and for crop protection.