The Hebrew term meaning "Repetition." The oral Torah of the Pharisees was not recorded in a set written form before the 2nd c. CE. Instead this body of tradition was preserved primarily through recitation & memorization. Standardization of the form & content of rabbinic tradition became necessary after the destruction of the temple in Jerusalem (70 CE). For the conscious mission of the rabbinic Academy was not only to preserve but to promote the authority of its interpretation of Torah for Jews everywhere. The Pharisaic principle of consensus was invoked to make the Academy's majority opinions the basis of normative Jewish religious observance. To promote further consensus, rulings were codified by topic & committed to memory by regular repetition. Thus, the leading rabbis of the Academy came to be referred to as the tannaim ["repeaters"].

While there are some indications that first century rabbis made use of written materials, none of these have survived. The impetus to record rabbinic oral tradition can be traced to Rabbi Aqiba ben Joseph, who became a scholar only after the age of 40. His work was continued by his disciple Rabbi Meir. But the publication of the Mishna in its definitive form (ca. 190 CE) is credited to Judah ha Nasi, who in its pages and thereafter is identified simply as Rabbi ["My Master"]. Yet, in its final form the Mishna still is a collegial project, presenting a compendium of consensus opinions on essental Jewish tradition rather than the teaching of just one scholar. It frequently juxtaposes contrasting opinions of tannaim whose teaching was regarded as reliable without declaring one or the other wrong, leaving plenty of room for further debate. 

Since the Mishna was a textbook for later rabbinic discussion of Jewish law, the opinions of the rabbis were organized systematically by topic and recorded in six general tractates rather than in the chronological order of their composition. This, coupled with the fact that the Mishna preserved little background information about the rabbis it cites, makes it difficult to identify the exact historical circumstances of its rulings. Some reflect the resolution of issues that are traceable to the time of Jesus; others, however, deal with debates that emerged in the post-temple era.  Most of our biographical information about the tannaitic sages comes from anecdotes told by subsequent generations of rabbis in commentaries on the Mishna recorded in the Palestinian & Babylonian Talmud.

[Edition used: Bladina, Philip, ed. Mishnayoth. 2nd ed. 7 vols. New York: Judaica Press, 1963-1964.]

Other resources on line:

• Mishnah - detailed article in Wikipedia's web, with links to the Open Mishnah Project in Hebrew & English.

• Mishnah - Eliezer Siegal's annotated image map of a page (U of Calgary).

• The Structured Mishnah -  Moshe Kline's new color-coded electronic edition of the Hebrew text (Jewish Theological Seminary).