We review some morphological aspects shared by all allografts. The main points are: 1) The expression of antigens of histocompatibility, essentially DR, allows the diagnosis of acute rejection with a significant specificity; 2) A lesion similar to "Quilty" effect may be seen in other allografts, and it is the first manifestation of an acute rejection; 3) Fine-needle aspiration biopsy may help in the follow-up of kidney transplant but has been demonstrated ineffective in other transplants; 4) The morphology of rejection is different according to the type of immunosuppressive therapy; 5) The basic lesions of chronic rejection are vascular, but chronic rejection may be diagnosed without vessels in the biopsy specimen, by the degree of atrophy of the parenchyma; 6) For the characterization of lymphomas in transplanted patients is necessary study the clonality of tumor cells, the gene rearrangement and the lymphoid subset; 7) Molecular Pathology may help in the early diagnosis of viral infections after transplantation.