This talk considers the presence of a rigid inclusion in the interior of a lipid membrane, which is a model for membrane-active proteins or peptides. The ensuing structural perturbation of the lipid chains can mediate a variety of interactions between inclusions or even destabilize the membrane. This topic is discussed on the basis of a molecular-level chain-packing approach and phenomenological elasticity theory. Particular focus is placed on models for membrane-spanning and amphipathic peptides. For the latter, we suggest a physical mechanism of forming pores by a membrane-induced self-assembly process.