This chapter discusses the intestinal morphology and the main transport mechanisms for internalization of nanosystems. The intestinal epithelium is responsible for the control of the passage of drugs, through digestion, absorption of nutrients and water homeostasis. In vitro tissue‐based models, also called ex vivo, are used in intestinal permeability studies in a way to move from in vitro to in vivo models. In this type of model, living functional tissues or organs are cultivated in a controlled environment after being isolated from organisms. Even though in vitro, ex vivo and in situ models keep improving, in vivo models are always necessary in drug delivery. The release kinetics and biodistribution are two main features tested through these techniques since they can contribute to the failure of therapeutic effects of nanosystems.