By definition exclusive monotheism is incompatible with polytheism, the belief that multiple gods exist. Note that the belief in multiple gods does not imply the worship of multiple gods. Many polytheists believe in the existence of many gods, but worship only one. This variant of polytheism is termed henotheism.
Western religions (Judaism, Christianity, Islam) traditionally held to exclusive monotheism, though an increasing number of adherents of these religions today subscribe to the inclusive monotheist view.
The Christian belief in the Trinity is traditionally considered a form of monotheism, although many Jews, Muslims and non-Trinitarian Christians would question this classification.