Rheological properties of model colloidal and non-colloidal suspensions were investigated. Colloidal bidisperse suspensions were studied with rotational rheometry and monodisperse suspensions studied with Stokesian dynamics simulations at high volume concentrations. Capillary and rotational rheometry was used in non-colloidal suspensions studies. Viscosity minimum for bidisperse suspensions was found when fraction of small particles was 40%. Numerical simulations show that increasing electrolyte concentration decreases the viscosity of the suspension, which was contributed to the reduction of the hydrodynamic radius of the particles. Non-colloidal hollow glass sphere suspensions were measured with the rotational rheometry and data could be fitted with Krieger-Dougherty model. Capillary rheometry results showed apparent shear thinning behavior.