502.15 - Phasing
502.15a - Phasing is a static ability that modifies the rules of the untap step. [CompRules 2003/07/01]
502.15b - During each player's untap step, before the active player untaps his or her permanents, all permanents with phasing the player controls phase out. Simultaneously, all objects that had phased out under that player's control phase in. (See Rule 217.8, "Phased Out," and Rule 302.1.) [CompRules 2005/08/01]
502.15c - If an effect causes a player to skip his or her untap step, the phasing event simply doesn't occur that turn. [CompRules 2003/07/01]
502.15d - Permanents phasing in or out don't trigger any comes-into-play or leaves-play abilities, and effects that modify how a permanent comes into play are ignored. Abilities and effects that specifically mention phasing can modify or trigger on these events, however. (Because no player receives priority during the untap step, any abilities triggering off of the phasing event won't go onto the stack until the upkeep step begins.) [CompRules 2005/10/01]
502.15e - When a permanent phases out, all damage dealt to it is removed. [CompRules 2003/07/01]
502.15f - A card that returns to play from the phased-out zone is considered the same permanent it was when it left. This is an exception to Rule 217.1c, which stipulates that a permanent "forgets" its previous existence when it changes zones. [CompRules 2003/07/01]
502.15g - Effects with limited duration and delayed triggered abilities that specifically reference a permanent will be unable to further affect that permanent if it phases out. However, other effects that reference the permanent (including effects with unlimited duration) can affect the permanent when it returns to play.
Example: A creature is affected by Giant Growth and then phases out during the same turn. If the creature phases back in somehow before the turn is over, it won't get the +3/+3 bonus from the Giant Growth because its effect has a limited duration. [CompRules 2003/07/01]
502.15h - Phased-out cards "remember" their past histories and will return to play in the same state. They "remember" any counters they had on them, any choices made when they first came into play, whether they were flipped when they left play, and whether they were tapped or untapped when they left play. They also "remember" who controlled them when they phased out, although they may phase in under the control of a different player if a control effect with limited duration has expired. [CompRules 2005/10/01]
Example: Diseased Vermin reads, in part, "At the beginning of your upkeep, Diseased Vermin deals X damage to target opponent previously dealt damage by it, where X is the number of infection counters on it." If Diseased Vermin phases out, it "remembers" how many counters it has and also which opponents it has previously damaged. When it phases back in, it will still be able to target those opponents with its upkeep-triggered ability. [CompRules 2003/07/01]
502.15i - When a permanent phases out, any Auras, Equipment, or Fortifications attached to that permanent phase out at the same time. This alternate way of phasing out is known as phasing out "indirectly." An Aura, Equipment, or Fortification that phased out indirectly won't phase in by itself, but instead phases in along with the card it's attached to. [CompRules 2007/05/01]
502.15j - If an Aura, Equipment, or Fortification phased out directly (rather than phasing out along with the permanent it's attached to), then it "remembers" the permanent it was attached to and returns to play attached to that permanent. If an Aura phases in and the permanent has left play or is no longer legal to enchant, the Aura returns to play and then is placed in its owner's graveyard. This is a state-based effect; see Rule 420. If an Equipment or Fortification phases in but it's no longer legal for it to be attached to the permanent it was attached to, the Equipment or Fortification returns to play and then stays in play, not attached to anything. This is a state-based effect; see Rule 420. [CompRules 2007/05/01]
502.15k - Permanents that phase in keep the same timestamps (see Rule 418.5d and Rule 418.5e) they had when they phased out. This doesn't change the fact that the permanents phase in simultaneously, however. [CompRules 2004/10/01]
502.15m - A permanent that phases in can attack and tap to play abilities whose cost includes the tap symbol ({Tap}) or the untap symbol ({Untap}) as though that permanent had haste. This applies even if that permanent phased out and phased back in the turn it came into play. The permanent remains able to do so until it changes controllers or leaves play. [CompRules 2008/10/01]
502.15n - A spell or ability that targets a permanent will resolve normally with respect to that permanent if the permanent phases out and back in before the spell or ability resolves. [CompRules 2003/07/01]
502.15p - Multiple instances of phasing on the same permanent are redundant. [CompRules 2003/07/01]
502.15.Ruling.1 - Permanents phasing out used to trigger leaves-play abilities, but as of 2005/10/01 they no longer do. [D'Angelo 2005/10/10]
Note - Also see Rule G16.10, "Phasing".

2001.

During each player's untap step, before the active player untaps his or her permanents, all permanents with phasing the player controls phase out. Simultaneously, all objects that had phased out under that player's control phase in. (See Rule 217.8, "Phased Out," and Rule 302.1.) [CompRules 2005/08/01]

2001.