Orthodox Judaism is the collective term for the traditionalist and theologically conservative branches of contemporary Judaism. Theologically, it is chiefly defined by regarding the Torah, both Written and Oral, as revealed by God to Moses on Mount Sinai and faithfully transmitted ever since.

Orthodox Judaism, therefore, advocates a strict observance of Jewish law, or halakha, which is to be interpreted and determined exclusively according to traditional methods and in adherence to the continuum of received precedent through the ages. It regards the entire halakhic system as ultimately grounded in immutable revelation, and beyond external influence. Key practices are observing the Sabbath, eating kosher, and Torah study. Key doctrines include a future Messiah who will restore Jewish practice by building the temple in Jerusalem and gathering all the Jews to Israel, belief in a future bodily resurrection of the dead, divine reward and punishment for the righteous and the sinners.

Orthodox Judaism is not a centralized denomination. Relations between its different subgroups are sometimes strained, and the exact limits of Orthodoxy are subject to intense debate. Very roughly, it may be divided between Haredi Judaism, which is more conservative and reclusive, and Modern Orthodox Judaism, which is relatively open to outer society. Each of those is itself formed of independent communities. Together, they are almost uniformly exclusionist, regarding Orthodoxy not as a variety of Judaism, but as Judaism itself.

While adhering to traditional beliefs, the movement is a modern phenomenon. It arose as a result of the breakdown of the autonomous Jewish community since the 18th century, and was much shaped by a conscious struggle against the pressures of secularization and rival alternatives. The strictly observant and theologically aware Orthodox are a definite minority among all Jews, but there are also some semi- and non-practicing individuals who affiliate or identify with Orthodoxy. It is the largest Jewish religious group, estimated to have over 2 million practicing adherents, and at least an equal number of nominal members.