This thesis is intended to provide a scientific and engineering understanding. of the rheology of polymer/gas solutions, which is essential for the production of polymeric foams. The presented research includes the design and construction of an innovative device capable of measuring the rheological properties of polymer/gas solutions under processing conditions. The proposed device is attached to a foam extrusion setup and is comprised of a rheological die, a variable resistance valve, and a small resistance nozzle. Two rheological dies of different geometry (capillary and slit dies) have been designed, manufactured, and experimentally verified. Several polymer materials were selected in order to test the designed equipment:​ two types of polybutylene succinate (PBS) and four types of polypropylene (PP) resins. Carbon dioxide (for PBS) and butane (for PP) were used as blowing agents. Care was taken to completely dissolve the gas in the polymer matrix and to maintain a single-phase polymer/gas solution during experimentation. Experiments were conducted at various shear rates, temperatures, and blowing agent concentrations. A rotational rheometer was used to obtain low shear rate viscosity data for the pure polymers. The apparent shear viscosity was converted into the true shear viscosity and successfully modeled by a combination of the generalized Cross-Carreau and Arrhenius equations in order to obtain unique material characteristics. Extensional viscosity of the polymer/gas solutions was analyzed on the basis of the pressure drop at the entrance to the slit die. The exit pressure data were utilized to demonstrate the presence of normal stresses in the flowing polymer melts.