Abstract: Many drugs, displaying a wide range of structures and diverse applications, can cross or bind to lipid membranes. Quantitative understanding of membrane interactions is thus important for several therapeutic approaches. First, membrane permeabilization represents the dominating mode of action of antimicrobial peptides (AMPs) and their synthetic mimics (SMAMPs). In terms of clinical applications, selectivity for bacterial over mammalian membranes is as important as good activity. Second, membrane interactions might influence loading, retaining, and releasing drugs from lipid-based drug delivery systems in a time controlled and targeted manner. Understanding the binding behaviour of the peptide drug exenatide to lipid membranes is not only important for characterization of its release from vesicular phospholipid gels, but might also help to understand other complex peptide-lipid interactions. The main aim of this thesis was to derive a mechanistic understanding of interactions of peptides and their analogues with model lipid membranes with a focus on the lipid composition of a membrane. Membrane permeabilization induced by AMPs and SMAMPs was quantified by a lifetime-based leakage assay using calcein-loaded vesicles. Different leakage behaviours were identified by considering active concentrations, strengths of individual leakage events, L1, and cumulative kinetics. Further experiments using isothermal titration calorimetry (ITC), monolayer adsorption measurements, and differential scanning calorimetry (DSC) helped to characterize the initial binding of AMPs and SMAMPs to lipid membranes and their propensity to induce electrostatic lipid clustering. Leakage experiments showed that the leakage behaviour changes with both, the structure of the AMP or SMAMP and the lipid composition of the membrane. The activity seems to increase if a membrane-active agent favours a permeabilization mechanism to which the particular lipid composition is especially susceptible. A closer look at kinetic profiles allowed distinguishing leakage induced by asymmetric stress from leakage events that occur stochastically. Very hydrophobic and unselective compounds seem to act mainly by hydrophobically driven asymmetry stress, especially when acting on zwitterionic phosphatidylcholine (PC) membranes. This mechanism brings about poor selectivity because all lipid membranes (bacterial and mammalian) contain a hydrophobic core. Stochastic leakage events, on the other hand, probably depend more on lipid compositions. Negatively charged lipids like phosphatidylglycerol (PG) or cardiolipin (CL) triggered the initial electrostatic attraction of polycationic AMPs or SMAMPs to bacterial membranes. High amounts of phosphatidylethanolamine (PE) seem to counteract the unselective asymmetry stress mechanism. Finally, especially strong leakage events were induced in vesicles containing CL. In this way, compounds that induce only rare leakage events might still be effective. In the second part of the thesis, an ITC fit model was introduced to study complex peptide-lipid interactions based on primary binding of peptide to the lipid layer and secondary binding to pre-bound peptide. Exenatide served as an exemplary peptide that interacts electrostatically with mixed POPC/POPG liposomes and self-associates at Kd = 46 μM. A global fit of various ITC curves revealed that exenatide binds primarily to a binding site at the outer membrane leaflet composed of 2-3 negatively charged POPG and some POPC molecules. Primary binding showed high affinity with a Kd1 of 0.2-0.6 μM, while secondary binding with a Kd2 of 10-46 μM was weaker. ITC was able to quantify primary and secondary binding separately, based on different binding enthalpies. Unlike ITC, other methods such as tryptophan fluorescence and microscale thermophoresis (MST) probably represent only a summary or average of both effects. Many similar ITC data can be found in literature that possibly involve primary and secondary binding effects. Those data could benefit from a fit model as presented in this thesis