Product demand is an important component of business research. The formulation of demand functions is fundamental in model setups, and can be a direct factor in model results. Demand dependency is an important area in the study of demand functions. This dissertation fits in the demand dependency literature, where many interesting industrial phenomena have been explored, but many others have not yet been studied. In addition, the existing literature mainly focuses on pricing models. Very little has been done on quality design and analysis. This dissertation includes quality as a decision variable in various demand dependency models, investigates some interesting business scenarios, and provides insights to many managerial issues.