A siddur (Hebrew: סדור‎; plural סדורים siddurim) is a Jewish prayer book, containing a set order of daily prayers. This article discusses how some of these prayers evolved, and how the siddur, as it is known today has developed. A separate article, Jewish prayer, discusses the prayers that appear in the siddur, and when they are said.

The earliest parts of Jewish prayer book are the Shema Yisrael ("Hear O Israel") (Deuteronomy 6:4 et seq), and the Priestly Blessing (Numbers 6:24-26), which are in the Torah. A set of eighteen (currently nineteen) blessings called the Shemoneh Esreh or the Amidah (Hebrew, "standing [prayer]"), is traditionally ascribed to the Great Assembly in the time of Ezra, at the end of the Biblical period.

The name Shemoneh Esreh, literally "eighteen", is an historical anachronism, since it now contains nineteen blessings. It was only near the end of the Second Temple period that the eighteen prayers of the weekday Amidah became standardized. Even at that time their precise wording and order was not yet fixed, and varied from locale to locale. Many modern scholars believe that parts of the Amidah came from the Hebrew apocryphal work Ben Sira.