Aspiration-induced pneumonia is an important cause of death in aging society. Improper viscosity and/or cohesiveness of bolus lead to serious problems. Since chemical factors such as taste and odor, temperature in addition to textural properties as well as physiological and psychological condition of subjects influence the mastication and deglutition processes, comprehensive approach is necessary to solve the problem. Rheological changes in the ingested foods in the oral cavity, and in the subsequent transportation to oropharyngeal region then to esophagus are described. Then, recent studies on the modification of rheological properties of bolus by hydrocolloids are also described