The present invention relates to a biostable polyurethane or polyurea comprising: (a) a soft segment comprising a polysiloxane of the general formula (I); and (b) greater than 0 and less than 40 wt % of a hard segment which is a reaction product of a diisocyanate and a linear difunctional chain extender, processes for their preparation and their use in the manufacture of biomaterials, devices, articles or implants.