Differences in recognition of physical properties of food in food science and engineering and the influence of water on them were discussed. The physical properties, 'Bussei' in Japanese, can be usually defined in food engineering and physics as 'the physical quantities that characterize a substance and do not depend on the shape and size of the material'. There seem, however, to be other interpretations of the physical properties among food researchers and technologists. For example, some researchers such as food chemists and cooking scientists refer to the dynamic properties and the texture of foods as the 'Bussei', whereas others such as technologists in food companies refer to it as the physical characteristics of foods reflecting some physical phenomena in food manufacturing and preservation processes. Most of the latter two types of 'Bussei' are influenced by the size and shape of materials and are not, therefore, the true physical properties. The type of 'Bussei' useful for food researchers and engineers would, however, vary depending on the situation or problem to be solved. Physical properties of foods are used for several purposes: first, they are indispensable parameters in the engineering models for predicting the optimal conditions for food manufacturing; second, the inner structure or state of food materials can be estimated from the behaviors of some of their physical properties. For example, water interacts with many food components; and thus the amount and/or state of water considerably influences the physical properties of foods, for example, by causing a change in the dynamic properties during sol-gel transition and the glass-rubber transition by the plasticizing effect of water.