Amphiphilic molecules form micelles and complexes with proteins in solutions to provide unique functions, utilized in various fields of chemistry and biology, and multiple association-dissociation equilibria, governing the micellization and complexation, are key issues in these fields. The present feature article first presents an overview of theoretical backgrounds of the multiple association-dissociation equilibria in solutions of amphiphiles and then applies the theoretical results to the following four solution systems of amphiphilic molecules to understand their micellization and complexation behavior: the reverse micelle of the polymer living anion, the hydrophobe-uptake spherical micelle, the cylindrical micelle of surfactants, and the protein-surfactant complex.