C.3.1- At the start of this step, the current player declares any creatures he or she wants to attack with this turn. If no attackers are declared, the game skips the rest of this step, all of steps 3 and 4, and goes directly to the End of Combat step (see Rule C.6). This is called a "null attack". If a legal attack is declared, first place any triggered abilities which triggered on the attack declaration on the stack, then the current player receives priority to play spells and abilities. [CompRules 1999/04/23]
C.3.2- You declare a single opponent (not any of your opponent's creatures) to attack and your entire attack applies to this one opponent. You cannot attack yourself or your own creatures. [D'Angelo 1999/05/01]
C.3.3- Only creatures the current player controls can attack. Creatures which are tapped or of creature type Wall cannot attack. [CompRules 1999/04/23]
C.3.4- Creatures which the current player did not control continuously since the beginning of the turn cannot attack. [CompRules 1999/04/23] See Rule G.37.
C.3.5- Declaration of attackers is simultaneous, not sequential, and it does not go on the stack. [CompRules 1999/04/23]
C.3.6- Tapping the creature is not considered a cost to attack. This does take place, however, just before any attack costs would be paid. [CompRules 2002/02/20 - 308.2]
C.3.7- An attack declaration may have additional costs associated with it. If these costs include a mana payment, then the current player may play mana abilities in order to make the payment. [CompRules 1999/04/23] For example, Propaganda imposes a mana payment on declaring attackers.
C.3.8- A creature becomes an attacking creature during the declaration (just after the attack is determined to be valid) and remains an attacking creature until removed from combat or until the end of the phase. [CompRules 1999/04/23] [bethmo 1999/10/05]
C.3.9- If there are creatures that must attack, the attack declaration is illegal if any subset of the creatures that were required to attack but were left out could have been added to the set of attackers. [CompRules 2001/07/23 - 500.2]
C.3.Ruling.1- Players are not allowed to declare an illegal attack. If a player announces a creature which cannot attack, or some combination of creatures which is not legal, or if a player cannot pay any costs associated with the attack, then that attack declaration is ignored and the player gets a chance to try again. [CompRules 2001/07/23 - 500.2]
C.3.Ruling.2- Effects can prevent a creature from attacking. For example, Island Sanctuary. [CompRules 2001/07/23 - 500.1]
C.3.Ruling.3- Creatures which have the ability to not tap when attacking, must still be untapped during this step in order to be declared as an attacker. [CompRules 1999/04/23]
C.3.Ruling.4- Creatures with a zero power can still attack. [D'Angelo 1999/05/01]
C.3.Ruling.5- You only check if the creature is allowed to attack (such as can only attack if opponent has Islands) during this step. If any attack enablers are removed or attack inhibitors are introduced later, it does not make a difference. The creature is still attacking. [CompRules 1999/04/23]
C.3.Ruling.6- If you have two creatures with "This creature can't attack unless another creature attacks", then you can declare both of them and have them meet each other's requirement. [CompRules 1999/04/23]
C.3.Ruling.7- An ability which involves "attacking alone" or "blocking alone" refers to having the creature be the only attacker or only blocker. For example, Reckless Ogre reads "Whenever Reckless Ogre attacks alone, it gets +3/+0 until end of turn." and the ability triggers when it is the only attacker. [WotC Rules Team 2000/01/11]
Note- Banding of attackers must be declared at the same time attackers are declared and cannot be changed later. See Rule A.8.3. [D'Angelo 1998/02/03]
Note- Players do get a chance to play spells and abilities during this step after attackers are declared. This is an ideal time for the defending player to eliminate any attackers they do not want to deal with, using Incinerate for example. It is also a good time to enhance blockers with spells like Jump so they can be used for blocking. [D'Angelo 1999/05/01]
Note- Creatures which are destroyed are removed from the combat even if they are regenerated. See Rule C.1.6. [D'Angelo 1998/02/03]