- Kitniyot, qit'niyyoth (Hebrew: קִטְנִיּוֹת ,קטניות , קיטניות) (literally little things) are a category of foods defined by Jewish law and tradition which Ashkenazi Jews (Jews from Eastern Europe, Germany, etc.) refrain from eating during the Biblical festival of Passover.
- The Torah (Exodus 13:3) prohibits Jews from eating leaven (chametz) during Passover. Technically, chametz is only leaven made from the "five grains": wheat, spelt, barley, shibbolet shu'al (two-rowed barley, according to Maimonides; oats according to Rashi) or rye, although there are additional rabbinic prohibitions against eating these grains in any form other than matzo.
- The earlier Poskim mention that rice, buckwheat/kasha, millet, beans, lentils, peas, sesame seeds and mustard are included in the minhag 
- On the other hand, potatoes (see below), coffee, tea, garlic, nuts, radishes and olives and not treated as kitnios 
- Iggeros Moshe assumes that peanuts are not kitnios but notes that some have a custom to be machmir. 
- Among traditional Ashkenazi Jews, the custom during Passover is to refrain from not only products of the five grains but also kitniyot. Literally "small things," such as other grains and legumes. Traditions of what is considered kitniyot vary from community to community but generally include maize (North American corn) , as well as rice, peas, lentils, and beans. Many also include peanuts in this prohibition, and one source, the Chayei Adam, also includes potatoes in his list, although his opinion is not followed by any large or major groups. Sephardi Jews typically do not observe the ban on kitniyot, albeit some groups do abstain from the use of dried pulses during Passover.
- for a list of kitniyot see http://www.kashrut.com/Passover/kitniot_list/ 
Reason for prohibition
- The Smak (Rabbi Yitzchak of Korbol) explains that products of kitniyot appear like chametz products. For example, it can be hard to distinguish between rice flour (kitniyot) and wheat flour (chametz). Therefore, to prevent confusion, all kitniyot was prohibited.
- The Beit Yosef (Rabbi Yosef Karo, 16th century, Israel) notes that since regular grains may become mixed together with kitniyot (apparently due to changes in crop cycles), one may inadvertently come to eat actual chametz.
- The origins of this practice are not clear, though two common theories are that these items are often made into products resembling chametz (e.g. cornbread), or that these items were normally stored in the same sacks as the five grains and people worried that they might become contaminated with chametz. It was also possible that crop rotations would result in the forbidden chametz grains growing in the same fields, and being mixed in with the kitniyot. Those authorities concerned with these three issues suggested that by avoiding eating kitniyot, people would be better able to avoid chametz. The Vilna Gaon (Hagaos HaGra, ibid.) indeed actually cites a novel source for this custom. The Gemorrah in Pesachim (40b) notes that Rava objected to the workers of the Raish Gelusa (the Exilarch) cooking a food called chasisi on Pesach, since it was wont to be confused with chametz. The Tosefos explain that, according to the Aruch, chasisi are lentils, and thus, argues the Gra, establishes the basis for the concern of kitniyot. Rabbi David Golinkin in the Responsa of the Masorati (Conservative) Movement cites Rabbenu Manoah (Provence, ca. 1265) who wrote an opinion in his commentary on Maimonides (Laws of Festivals and Holidays 5:1) that "It is not proper to eat qitniyot on holidays because it is written (in Deut. 16:14) that ‘you shall rejoice in your festivals’ and there is no joy in eating dishes made from kitniyot".
Difference between chametz gamur and kitniyot
- Jewish law is customarily quite stringent about the prohibition against even tiny amounts of chametz in the house during Passover, much more so than the regular laws of kashrut. Thus a tradition developed to avoid these products altogether, and this eventually developed into what most of the European Jewish community accepted upon themselves as a minhag, a legally binding custom.
- While this practice is considered binding for Ashkenazim in Orthodox Judaism, these items are not chametz and therefore are not subject to the same prohibitions and stringencies as chametz. For example while there is a prohibition against owning chametz on Passover, no such prohibition applies to kitniyot. Similarly, while someone would not be permitted to eat chametz on Passover unless his life were in danger, the prohibition of kitniyot is not so strict. People who might be permitted to eat kitniyot include infirm people and pregnant vegetarians. Such dispensations are far more common in Israel where there is a large Sephardi population.
Halachot of kitniyot
- The minhag to not eat kitnios begins on Erev Pesach at the same time that one may not eat chametz . Although one may not eat kitnios, one may own and derive benefit from kitnios. Therefore, on Pesach one may keep cans of sweet corn in their property or feed millet to their parrot. Additionally, children, people who are ill, and people whose diet is otherwise restricted and must eat kitnios, are excluded from the minhag and may do so after consulting with a Rav. This halacha is quite relevant to baby formulas and nutritional supplements (e.g. Ensure) which invariably contain kitnios, and are usually used by people who have few non-kitnios choices, if any. When such foods are used on Pesach they should be prepared in special non-Pesach and non-chametz utensils, which should not be washed with the Pesach dishes. 
- Kitnios is batel b’rov, which means that if someone accidentally put kitnios into their Pesach food, the food is b’dieved permitted assuming the food contains more non-kitnios than kitnios . This means that although the food may have a pronounced taste of kitnios, the food is permitted (unless there are recognizable pieces of kitnios which haven’t been removed). Therefore, if a beverage is sweetened with aspartame made of kitnios shenishtaneh, even those people who hold that aspartame is forbidden (as explained above) may drink the beverage because the aspartame is batel b’rov in the other ingredients. Similarly, we have seen that there is a disagreement as to whether fenugreek is kitnios. Nonetheless, even those who follow the strict approach may consume maple syrup which is flavored with fenugreek (as it often is) because it is batel b’rov. Thus, although we’ve seen a number of disagreements as to whether certain foods are or aren’t kitnios, those disagreements are limited to one who wants to consume the actual item (or a hashgachah certifying someone else who is intentionally putting the ingredient into a food), but these disagreements rarely affect consumers. 
- Even where the prohibition of kitniyot was practiced, it was not without opposition. Some poskim went as far as to call it a "stupid practice" without basis. Others, including Rav Moshe Feinstein did not advocate abandoning the custom, but he certainly opposed the tendency to expand the list of forbidden kitniyot (see Igrot Moshe, Orah Hayyim 3. 63).
- Sephardic and Yemenite Jews generally do not accept the need for this minhag, and thus eat kitniyot on Passover. Some Ashkenazi Jews in Israel who have married Sephardic Jews adopt the Sephardic custom; this often occurs with Orthodox rabbinic approval — a noted leniency, since Orthodox rabbis usually hold that one may not reject the minhagim (customs) of one's parents. In light of the gathering of Jews of all ethnic groups back in the land of Israel, Masorti Jews, the Conservative movement in Israel, hold that all Jews living in Israel may safely abandon the minhag of refraining from kitniyot.
- Outside Israel however, the distinction between Kitniyot and non Kitniyot or Chametz can be almost academic. Instead of relying on reading ingredients on labels, Orthodox Jews require their manufactured and processed food to be certified or checked (year round) to see that it qualifies with Halacha in both the ingredients and tools used to process and manufacture the food. For Passover, Jews require certification of this in addition to certification that foods neither contain chametz nor were prepared with utensils that contain chametz. In most countries outside Israel (including the US) the major agencies which certify food as kosher, or kosher for Passover, do not have a special category for food which is "only kitniyot" and not chametz. Therefore, it is nearly impossible for the typical consumer to tell conclusively if an item is kitniyot or would be considered chametz because of some hidden ingredients or manufacturing process. Thus, those who would eat kitniyot—the sick, sephardim, or others who could take advantage of the special category known as kitniyot—are largely out of luck and must treat any processed items without full certification as possibly full fledged chametz
Derivatives of kitniyot
- There is one product called "quinoa" (pronounced "kin-O-ah," or keen-WA) that is the subject of much discussion. Although quinoa resembles a grain, it is technically in the "goose foot" family, which includes sugar beets and beet root. As such, some rabbis (for example, Rabbi Heinemann of Star-K) permit its use even for Ashkenazim on Passover, while other rabbis do not.
- Beis Yosef O.C. 453, Rema 453:1 & 464:1 and Mishnah Berurah 453:4, 7 & 11
- Sha’arei Teshuvah 453:1, Chayei Adam 127:7
- (O.C. III:63)
- Mishnah Berurah 453:4
- Shevet HaLevi III:31 citing Chok Yaakov 471:2
- Rema 453:1 as per Mishnah Berurah 453:9