Heterotransplantation of human cancer cells or tumor biopsies into immunodeficient rodents (xenograft models) has, for the past two decades, constituted the major preclinical screen for the development of novel cancer therapeutics. Despite limitations, these models have identified clinically efficacious agents, and remain the 'workhorse' of the pharmaceutical industry. However, if therapeutic approaches to treating tumors according to their molecular characteristics are to be achieved, additional new models of human cancer will be required to represent the genetic diversity that exists within tumor histologies. This protocol details a method for establishing xenografts from primary solid-tumor isolates or cells grown in culture. The procedure relies on immunodeficient mice to provide a host for the establishment of human xenografts. The procedure can be completed in 1-2 h with results being obtained in 1-4 months.