Although most testicular and paratesticular tumors can be recognized by their light microscopic features, some raise significant differential diagnostic questions. Immunohistochemical staining has proved of significant value in this situation. There is still a role for the traditional markers, including placental-like alkaline phosphatase and alpha-fetoprotein, but newer markers provide additional support and often have greater sensitivity and specificity for many diagnoses. OCT4 is virtually 100% sensitive and specific for seminoma, embryonal carcinoma, and intratubular germ cell neoplasia, unclassified type. Inhibin-alpha, among testicular tumors, is limited to those in the sex cord-stromal category or those having adrenocortical-type differentiation (testicular tumor of the adrenogenital syndrome) or of trophoblastic lineage. Calretinin is another positive marker for the sex cord-stromal tumors but has less specificity. Additional markers, including differential cytokeratins, c-kit, CD30, epithelial membrane antigen, S-100, melan-A, and others, are useful in specific situations. This article reviews the application of immunohistochemical markers for a number of differential diagnostic considerations in the testis and paratestis categorized according to their light microscopic patterns.