The interactions of the membrane-active peptides alamethicin and protegrin with lipid bilayers are studied. The influence of the bilayer composition on the orientation transition of alamethicin is studied using oriented circular dichroism. The makeup of the headgroup region of the bilayer is altered while the composition of the hydrocarbon chains remains constant. When a smaller lipid headgroup is added to the bilayer, the insertion transition shifts to higher peptide concentrations. This can be explained as a reduction of the energy cost of adsorbing the peptide in the bilayer due to the smaller lipid headgroup. The interactions of the beta-sheet peptide protegrin with model membranes are studied by oriented circular dichroism and lamellar x-ray diffraction. A transition between two states with distinct oriented circular dichroism spectra is observed which is a function of the peptide concentration and hydration. Lamellar x-ray diffraction indicates that protegrin produces membrane thinning in the same lipid system for peptide concentrations below those needed to bring on the transition. This leads to the conclusion that the low concentration and hydration state has protegrin adsorbed into the headgroup region of the bilayer parallel to the membrane surface. Comparisons between the behaviors of alamethicin and protegrins are drawn which suggests that the two peptides have the same mode of action.