Solid-state NMR is a powerful and versatile spectroscopic method for analyzing the sophisticated structure of biological systems, especially the non-crystalline and insoluble systems that are difficult for solution NMR and X-ray diffraction to study. In this thesis, we used solid-state NMR to elucidate the structures of complex systems, such as peptide aggregates, and membrane peptides in oriented and unoriented lipid bilayers. Various NMR techniques have been used to study the peptide-peptide and peptide-lipid interactions, which have a great influence on the topology of these systems.;The peptides we are interested in are antimicrobial peptides (AMP), which can kill microbes at micromolar concentration by disrupting the microbial cell membrane. These peptides usually contain cationic residues and have an amphipathic structure. Understanding the peptide-peptide and peptide-lipid interactions will shed light on the antimicrobial mechanism of AMP. To determine the oligomeric structure of the aggregates of an antimicrobial beta-hairpin peptide, Protegrin-1 (PG-1), we used 2D 1H-driven 13 C spin diffusion and other correlation methods. We found that PG-1 aggregates in a parallel fashion with like strands lining the intermolecular interface. In an oriented membrane system, we applied a method for determining the orientation of beta-sheet membrane peptides using 2D separated local-field spectroscopy. Retrocyclin-2, an antibacterial and antiviral beta-hairpin peptide, was found to adopt a transmembrane orientation in short-chain lipids (DLPC) and a more in-plane orientation in long-chain lipids (POPC), which indicates that the membrane thickness affects the peptide orientation. In unoriented membrane systems, we utilized a variety of methods under magic-angle spinning (MAS), such as 13C{lcub}31P{rcub} REDOR, 13C DIPSHIFT, LGCP and 1H spin diffusion, to study the interaction of PG-1 with lipid bilayers. The experimental results led to the toroidal pore model as the mechanism of action of PG-1. Moreover, we found that the guanidinium-phosphate complexation is the driving force for pore formation. Both ionic interaction and hydrogen-bonding play a significant role in stabilizing the guanidinium-phosphate complex, because altering either one of the two factors would affect the antimicrobial activity and membrane topology of PG-1 dramatically. By mutating the Arg sidechain with methylation, we showed that without sufficient hydrogen-bonding, the mutant adopts an in-plane orientation and undergoes fast uniaxially rotation.