Crystalline elastomers are olefin polymers or co-polymers, where the typical mechanical properties of crystalline materials (high rigidity and high stress-to-break) are associated with those of an elastomer (high deformability, high ductility). The above properties can be modulated by suitably changing the degree of order of the monomer distribution along the polymer chain. This has been made possible in recent years by the development of metallocene and post-metallocene homogeneous catalysts. In this paper we focus our attention on the relations between the rheological response of the molten polymers and their local microstructures. Examples will include: - syndiotactic Polypropylene and Poly(1-butene) (sPP and sPB) whose viscoelastic plateau modulus is shown to be strictly related to the degree of stereo-regularity of the polymer chains; - Isotactic polypropylene-block-polyethylene block copolymers (iPP-b-PE), which are characterized by a wide range of morphologies deriving from the melt immiscibility or by the crystallization of one or both blocks (breakout crystallization). In this case, rheological measurements of the viscoelastic behavior at low frequencies allow for the detection of various ordered mesophases.