(According to the Qur'an and Sunnah, as extracted and inferred by scholars of the Hanafi school.)
From "Mukhtasar al-Quduri", a matn of Hanafi fiqh

1.0 THE OBLIGATION OF FASTING

1. The time for fasting is from the rising of the second dawn until the setting of the sun.
   
2. Fasting is : abstention from eating, drinking and sexual intercourse by day with the intention.
   
3. If in Ramadan a child reached adulthood, or an unbeliever accepted Islam, they abstain [from things which invalidate fasting] for the remainder of that day, and fast that which comes thereafter. They do not make up what passed.
   
4. If a traveller arrives [at his place of residence], or a [menstruating] woman attains purity with part of the day [remaining], they abstain [from those things which invaliate fasting] for the rest of that day.

2.0 THE INTENTION

Fasting is of two sorts : obligatory and supererogatory (nafl).

1. The obligatory is of two sorts : among it is that which is attached to a specific time, such as the fast of ramadan, and a specified vow. The fasting of [this category] is valid with an intention from the night, but if one did not intend until the morning, the intention suffices him between [dawn] and {the middle of the day}. The second sort is that which becomes obligatory to fulfil, such as the make-up [fasts] of Ramadan, unrestricted vows, and atonements. These are not valid without an intention from the night.
   
2. All of the nafl is valid with an intention before {the middle of the day}.

3.0 SIGHTING THE CRESCENT

3.1 For Ramadan

1. It is imperative for the people to seek the new crescent on the twenty-ninth day of Sha`ban. Then, if they see it, they fast [the following day], but if it is obscured from them, they complete the couting of Sha`ban as thirty days and then fast [after that].
   
2. Whoever sights the new crescent of Ramadan alone fasts, even if the imam does not accept his testimony.
   
3. If there is some obstruction in the sky, the imam accepts the testimony of one upright [Muslim] - be that male or female, free-man or slave - for the sighting of the crescent. But, if there is no obstruction in the sky, [one individual's] testimony is not accepted until a large multitude sight it, by whose report [certain] knowledge is attained.

3.2 For `Id

1. Someone who alone sights the crescent for ending the fast does not stop fasting.
   
2. When there is some obstruction in the sky, only the testimony of two men, or one man and two women, is accepted for [sighting of] the crescent for ending the fast. But, if there is no obstruction in the sky, only the testimony of a large multitude -- by whose report [certain] knowledge is attained -- is accepted.

4.0 ACTIONS OF THE FASTING PERSON

4.1 Things that do not break the fast

1. If the fasting one ate, or drank, or had sexual inercourse out of forgetfulness [that he was fasting], his fast is not broken.
   
2. If he slept and then had an erotice dream, or looked at a woman and ejaculated, or oiled [his head], or underwent blood-letting, or used antimony [in his eyes], or kissed, his fast is not broken.
   
3. If one is overcome by vomiting, his fast is not broken.
   
4. If he applied drops inside his urethra, his fast is not broken according to Abu Hanifah. Abu Yusuf said : his fast is broken.

4.2 Things that are Disliked for the Fasting Person

1. If someone tastes something with his mouth, his fast is not broken, but it is disliked for him to do that.
   
2. It is disliked for a woman to chew the food for her infant if she has some alternative.
   
3. Chewing gum does not break the person's fast, but it is disliked.

4.3 Things that Break the Fast and require Makeup

1. If he ejaculated on account of a kiss or touch, then make-up is due upon him.
   There is no harm in kissing if he feels himself safe, but it is disliked if he does not feel safe.
   
2. Makeup is due, but not expiation, for someone who had intercourse in other than the private parts and ejaculated.
   
3. If one deliberately made himself vomit a mouthful then makeup is due upon him.
   
4. The fast of someone who swallows pebbles or iron is broken.
   
5. Whoever had an anal enema, or applied nose-drops, or ear-drops, or treated a torn belly or a skull-fracture with medicine such that it reached his body cavity or his brain, his fast is broken.
   
6. If someone had suhur thinking the dawn had not [yet] risen, or broke his fast thinking the sun had set, and then it turned out that the dawn had risen, or that the sun had not set, makes up that day, but there is no expiation due on him.
   
7. Someone who lost consciousness in Ramadan does not make up the day on which the loss of consciousness occurred, but he makes up that which came after it.
   
8. If an insane person regained sanity with part of Ramadan [remaining], he makes up what passed of it.
   
9. If a woman menstruates, she stops fasting and makes up [fasting for the days of menstruation].
   
10. Whoever enters into an optional fast, or an optional prayer, and then spoils it, makes it up.

4.4 Things that Break the Fast and require Makeup and Expiation

1. Expiation is due on someone who deliberately has sexual intercourse in one of the two passages, or eats or drinks something which provides nutrition, or is used for treatment
   
2. The expiation is like the expiation for zihar.
   
3. There is no expiation for spoiling a fast in other than Ramadan.

5.0 EXCUSES

5.1 Those who may Postpone Fasting

1. Someone who is sick in Ramadan, and fears that if he fasts his sickness will increase, breaks his fast and makes [it] up [later].
   
2. If one is a traveller who is not harmed by fasting, then for him to fast is preferable, but if he does not fast and makes it up [later] it is permissible.
   
3. The pregnant or nursing woman, if they fear for their children, do not fast and make it up, and there is no redemption due upon them.

5.2 Making up Missed Fasts

1. The makeup of Ramadan may be performed separately if one wishes, or consecutively if one wishes.
   
2. If one delayed it until another Ramadan entered, he fasts the second Ramadan, and makes up the first after it, and there is no redemption due upon him.
   
3. If the invalid or the traveller dies while they are in that condition, makeup is not incumbent upon them. But, if the invalid recovers, or the traveller takes up residence, and then they die, makeup is incumbent upon them for the extent of the health or residence.

5.3 Redemption (Fidyah)

1. The aged man who is not capable of fasting does not fast, and for every day he feeds a poor person, just as one feeds in expiations.
   
2. Whoever died with makeup [fasts] of Ramadan due upon him, and bequeathed for it, his guardian, on his behalf, feeds for every day to one poor person : half a sa` of wheat, or one sa` of dates, or one sa` of barley.

6.0 SECLUSION (I`TIKAF)

1. Seclusion is praiseworthy. It comprises remaining in the mosque, with fast and the intention of seclusion.
   
2. It is prohibited for the secluded one :

   	to have sexual intercourse
   	to touch [with lust]
   	to kiss

3. If the secluded one had sexual intercourse, by night or day, his seclusion is invalidated.
   
4. He should not exit from the mosque except for a a human need, or [for] Jumu`ah [prayer].
   
5. There is no harm in his buying or selling in the mosque without bringing the goods there.
   
6. He should speak only well, but [intentional] silence is disliked for him.
   
7. Whoever obligated upon himself seclusion for [a number of] days is obliged to to seclude himself for them along with their nights, and [the days] are consecutive, even if he did not stipulate consecutiveness.