Haredi Judaism consists of groups within Orthodox Judaism that are characterized by their strict adherence to halakha (Jewish law) and traditions, in opposition to modern values and practices. Its members are usually referred to as ultra-Orthodox in English; however, the term "ultra-Orthodox" is considered pejorative by many of its adherents, who prefer terms like strictly Orthodox or Haredi. Haredi Jews regard themselves as the most religiously authentic group of Jews, although other movements of Judaism disagree.

Some scholars have suggested that Haredi Judaism is a reaction to societal changes, including political emancipation, the Haskalah movement derived from the Enlightenment, acculturation, secularization, religious reform in all its forms from mild to extreme, the rise of the Jewish national movements, etc. In contrast to Modern Orthodox Judaism, followers of Haredi Judaism segregate themselves from other parts of society to an extent. However, many Haredi communities encourage their young people to get a professional degree or establish a business. Furthermore, some Haredi groups, like Chabad-Lubavitch, encourage outreach to less observant and unaffiliated Jews and hilonim (secular Israeli Jews). Thus, professional and social relationships often form between Haredi and non-Haredi Jews, as well as between Haredi Jews and non-Jews.

Haredi communities are found primarily in Israel (12.9% of Israel's population), North America, and Western Europe. Their estimated global population numbers over 1.8 million, and, due to a virtual absence of interfaith marriage and a high birth rate, the Haredi population is growing rapidly. Their numbers have also been boosted since the 1970s by secular Jews adopting a Haredi lifestyle as part of the baal teshuva movement; however, this has been offset by those leaving.