"Who is a Jew?" is a basic question about Jewish identity and considerations of Jewish self-identification. The question pertains to ideas about Jewish personhood, which have cultural, ethnic, religious, political, genealogical, and personal dimensions. Orthodox Judaism and Conservative Judaism follow Jewish law (Halakha), deeming people to be Jewish if their mothers are Jewish or if they underwent a halakhic conversion. Reform Judaism and Reconstructionist Judaism accept both matrilineal and patrilineal descent as well as conversion. Karaite Judaism predominantly follows patrilineal descent as well as conversion.

Jewish identity is the objective or subjective state of perceiving oneself as a Jew and as relating to being Jewish. Under a broader definition, Jewish identity does not depend on whether a person is regarded as a Jew by others, or by an external set of religious, legal, or sociological norms. Jewish identity does not need to imply religious orthodoxy. Accordingly, Jewish identity can be cultural in nature. Jewish identity can involve ties to the Jewish community. Orthodox Judaism bases Jewishness on matrilineal descent. According to Jewish law (halacha), all those born of a Jewish mother are considered Jewish, regardless of personal beliefs or level of observance of Jewish law. Progressive Judaism and Haymanot Judaism in general base Jewishness on having at least one Jewish parent, while Karaite Judaism bases Jewishness only on paternal lineage. These differences between the major Jewish movements are the source of the disagreement and debate about who is a Jew.