In many technological applications, particles are dispersed in fluids that display a complex rheological behavior (filled polymers, nanocomposites, paints etc). The presence of the particles further complicates the flow behavior. For example, shear flow can create particle chains in the flow direction and influence the macroscopic rheological response. Most studies in the literature have been focused on highly filled systems (typically, volume fractions higher than 10%), usually with polydisperse particles of irregular shapes. In contrast, few studies are conducted on the rheology of dilute or semi-dilute suspensions of monodisperse spheres and on the relative flow induced microstructure. In this Phd thesis the effect of viscoelasticity on the bulk rheological properties has been explored. The behavior of model suspensions composed of non Brownian, inertialess, rigid spheres immersed in Newtonian and viscoelastic matrices is investigated in the concentration range from 0 up to 10%, thus encompassing both the dilute and semidilute regimes. The flow-induced alignment of non colloidal particles in viscoelastic fluids is also investigated systematically in an attempt to quantify the alignment of the particles and correlate it with shear rate, particle sizes and gap size. Alignment only appears at shear rates above a critical value. After this, the degree of alignment increases with strain and particle size.It is also shown that, for our viscoelastic medium, confinement is the key parameter for this phenomenon.