In this paper, we review the role of the rheological properties at the cellular and macroscopic scale. At the cellular scale, the different components of the cell are described, and comparisons with other similar systems are made in order to state what kind of rheological properties and what constitutive equations can be expected. This is based on expertise collected over many years, dealing with components such as polymers, suspensions, colloids and gels. Various references are considered. Then we review the various methods available in the literature, which can allow one to go from the microscopic to the macroscopic properties of an ensemble of cells, in other words a tissue. One of the questions raised is: can we find different properties at the macroscopic level than the ones that we start with at the cellular level? Finally, we consider different biological materials which have been used and characterized, in order to classify them. Constitutive laws are also proposed and criticized. The most difficult part of modeling is taking into account the active part of cells, which are not just plain materials, but are living objects.