The aim of these studies was the investigation of bile salt-lipid-interactions and associated colloidal systems as well as the characterization of the vesicle-micelle-phase transition with the occurring aggregates, vesicles and mixed micelles, with respect to the type, size, structure and stability. The studies contribute to gain insight in the biophysical-biochemical nature of bile salt-lipid-interactions, to facilitate the understanding of the physiological processes of lipid digestion, and to obtain fundamental information for the development of innovative drug delivery systems in the colloidal size range. For realizing the physicochemical investigations, a combination of thermodynamic methods, isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC), as well as scattering techniques, dynamic light scattering (DLS) and small angle neutron scattering (SANS), was used. The studies included partitioning and solubilization experiments of binary bile salt-phospholipid-systems with sodium cholate or sodium deoxycholate as bile salt components and DPPC, soy bean phosphatidylcholine, DPPG, or DPPC/DPPG mixtures with molar ratios 3:1 and 1:1 as phospholipid components. The characterization of the micellization of sodium oleate and of the formation of mixed micelles with bile salts and the fatty acid salts sodium oleate and sodium palmitate was the basis for the partitioning and solubilization experiments of ternary bile salt/sodium oleate-DPPC-and bile salt/sodium palmitate-DPPC-systems. Additionally, the particle size of the aggregates of the vesicle-micelle-phase transition and the influence of temperature and ionic strength on the stability of the binary and ternary systems were determined.