We have investigated the interactions between two carrier peptides and model membrane systems as well as the conformational consequences of these interactions. Studies performed with lipid monolayers at the air-water interface have enabled identification of the nature of the lipid-peptide interactions and characterization of the influence of phospholipids on the ability of these peptides to penetrate into lipidic media. Penetration experiments reveal that both peptides interact strongly with phospholipids. Conformational investigations indicate that the lipid-peptide interaction govern the conformational state of the peptides. Based on the ability of both peptides to promote ion permeabilization of both natural and artificial membranes, we propose a model illustrating the translocation process. For MPG, it is based on the formation of a beta-barrel pore-like structure, while for Pep-1, it is based on association of helices.