The important roles of histamine in body physiology and various pathologic events have been well established, whereas new and exciting findings are still being uncovered. Histamine is not only the major mediator of the acute inflammatory and immediate hypersensitivity responses, but has also been demonstrated to affect chronic inflammation and regulate several essential events in the immune response. The diverse effects of histamine on immune regulation are due to the differential expression and regulation of four histamine receptors (HR) and their distinct intracellular signals. Differences in the affinities of these receptors are highly decisive on the biological effects of histamine and agents that target HRs. In particular, the discovery of reciprocal regulation of T cell activity by H1 and H2 receptor activation indicating histamine-cytokine cross-talk, as well as the characterization of the fourth histamine receptor (HR4) and its expression on numerous immune and inflammatory cells have prompted a re-evaluation of the actions of histamine. This accounts for a new potential for HR antagonists and agonists in targeting various immunopathologic conditions.