Hasidic Judaism, also known as Chasidism, originated in what is now Ukraine during the 18th Century when Rabbi Israel ben Eliezer sought to revive the religion, which he felt had developed in a way that placed too much emphasis on scholarship and academic study at the cost of the simple enjoyment of worship and spirituality. In doing so, he hoped to increase the sense of participation felt by uneducated and poor Jews - who in many cases were unable to devote so much time and energy to Torah (Jewish Bible) study and discussion of Jewish philosophy as those from more affluent and educated backgrounds - and as a result attract Jews who may have been feeling distant or cut off from Judaism back to the religion. Hasidism was remarkably successful in this aim, and spread throughout Europe very rapidly - by 1830, the vast majority of Jews in central Poland, Ukraine and Galicia (a region currently divided between Poland and Ukraine) were Hasidim.

One chief difference between the Hasidim and other branches of Orthodox Judaism is in the way that they view the rabbi who, in Hasidism, is usually known as the rebbe (outside of Hadsidic circles, the rebbe is sometimes known as a Grand Rabbi). While a rabbi must be an expert on many aspects of Judaism, a rebbe is seen in a far more spiritual way as a conduit to G-d. As a tzaddik (exceptionally righteous person), the rebbe has perfected his personal service to G-d to such a degree that he attains physical awareness of G-d's presence in daily life and an awareness or perception that transcends the normal boundaries of existence. This is believed to give him such a powerful insight into life that he is virtually able to see into the future, and in many cases he is believed to be able to affect divine providence allowing the working of "miracles."

Hasidism also places great emphasis on the importance of the Kabbalah and believes that it can be taught to all through prayer - in many of Judaism's other denominations, Kabbalah is commonly considered of less importance or excessively esoteric and mystical for widespread study by all.

Hasidic prayers are accompanied by nigunim (singular nigun), melodious hummed tunes, which in some cases are based on very old East European folk tunes that have been adapted to give them a spiritual aspect or on sections of Torah text. Nigunim have been adopted by other Jewish groups, but it was the Hasidim who used them first. The tune of each nigun - some of which are specific to particular Hasidic communties - reflects the emotional tone of each prayer. Mental concentration during prayer is essential within all of those Jewish denominations that pray (Humanistic Jews do not), but this is taken to an altogether higher level by Hasidim who often devote a very great deal of time to each prayer - some groups spend seven seconds contemplating each and every word of the Amidah, a long prayer consisting on 19 blessings. Hasidim also place great importance on the mikvah, a pool of water within which Jews ritually immerse themselves to achieve spiritual cleanliness. Whereas many Orthodox Jews will visit a mikvah before religious holidays, many Hasidim will do so daily.

Followers of Hasidic Judaism may join groups known as Hasidic Dynasties, a sort of study and prayer group that has a rebbe as its spiritual leader, with leadership passing on after the rebbe's death - usually to one of the rebbe's family. In most cases, the dynasty will be named after the town in Eastern Europe where the rebbe or his family originated or where the movement began, such as the well-known Chabad-Lubavitch which, for over a century, was based in the Russian town Lyubavichi. Chabad-Lubavitch, which maintains "Chabad houses" on many university campuses to offer support to both Jewish and non-Jewish students, is notably different to other forms of Judaism and Hasidism in that it conducts outreach campaigns intended to draw non-observant and non-Orthodox Jews back to the faith and encourage them to adopt Orthodox observance - a practice known as kiruv which, in the words of the last Chabad Rebbe Menachem Schneerson, will "accelerate the coming of the Messiah." Outreaching is exceedingly rare in Judaism, which often actively seeks to discourage converts, but has led to Chabad becoming one of the largest Hasidic dynasties with around 200,000 adherents worldwide.