Molecular crowding and confinement in small spaces are remarkable features of living cells. To reveal the physicochemical aspects of these features, the effects of confinement-size and droplet surface on them should be separated from them. Water-in-oil microdroplets covered with a lipid layer (droplet system) have suitable characters to separate them, because the droplet sizes and lipid species covering the droplet are easily controlled. Here, we introduce recent achievements using the droplet system that has revealed effects of confinements on protein expression, accelerated phase-separation of biopolymer blends, and behaviors of highly concentrated biopolymers.