Proteins are the vital molecules in living cells and important targets for pharmaceutical and biotechnological applications. To function, they need a defined three dimensional structure. All the information necessary for a protein to achieve this conformation is encoded in its amino acid sequence. To understand the process by which a polypeptide chain folds into its three-dimensional structure (the so-called "protein folding reaction") is an essential element in structural biology. This work mainly focusses on the structural and functional characterization of partially folded intermediate states, which can accumulate during the folding process. Ankyrin Repeat proteins were choosen as model proteins, since their architectural simplicity allows the dissection of energetics to different structural elements.