A reshuffle is when a player turns over their discard pile and shuffles it to replace their empty deck. The player has a limited amount of control over when that happens, and effectively exploiting that control to is an important element of tactical play.
 Official Rules
- If you have to do anything with your deck - for example draw, look at, reveal, set aside, discard, or trash cards - and you need more cards than are left in your deck, first shuffle your discard pile and put it under your deck, then do the thing.
- If there are still not enough cards, you do the thing with however many cards you can. If when shuffling there are no cards in your deck, the shuffled discard pile simply becomes your new deck.
- When your deck is empty, you do not shuffle until you need to do something with cards from your deck.
- If you have to put a card on top of your deck when it is empty, that card becomes the only card in your deck.
 Other rules clarifications
- For cases where you don't know how many cards you are accessing (e.g. Library), you are doing them one at a time, and don't shuffle until you have no cards left in your deck and need to access another one.
- The –1 Card token applies as the first step of "draw N cards." This means you count it as if it were a card when determining when to reshuffle before drawing cards. This only applies to drawing cards, not other methods of accessing them.
- When shuffling Stash cards in your discard pile, you may only place those Stash cards anywhere among the shuffled cards; they cannot be placed on top of or in between any remaining cards in your deck. Also, you may look at any remaining cards in your deck before choosing where to place the Stash cards in your shuffled discard pile.
- You play a Smithy and need to draw 3 cards. However, you only have 2 cards in your (draw) Deck (and some other cards in your discard pile). In this case, you would reshuffle your discard pile, put it face down under the 2 cards in your deck, then you would pick up the 3 cards cards you need from your new deck (the 2 old cards and the top of the shuffled cards). You would not draw 2 cards first and then reshuffle and then take the top card of the new deck (though this makes no practical difference except with Stash).
- Witch is in the kingdom and your opponent has bought one. After your turn, you have 5 cards in hand and none in your draw deck, and several miscellaneous cards in your discard pile. One of the cards you have in hand is a Smithy. However, you cannot choose to reshuffle until your turn when you play Smithy and are required to draw cards.
In this (second) example, the timing of the reshuffle has real, practical consequences. If your opponent plays Witch (barring other conditions), you will gain a Curse to your discard pile; if your discard has already been reshuffled (when it shouldn't have been) into your new deck, you won't have the chance of drawing this Curse from Smithy. That's a distinct advantage to you.
Knowing when to reshuffle (or, perhaps more importantly, when not to) is a crucial part of advanced Dominion play. If you reshuffle during your Cleanup phase, then the all the cards you used that turn will be in the discard pile that gets reshuffled and will return to your draw pile immediately to be reused again relatively soon. If you reshuffle earlier in the turn, any cards currently in your hand or in play will be discarded after you shuffle, meaning they won't return to your draw pile until after the next reshuffle; cards this happens to are after described as missing the reshuffle. If you play draw cards unthinkingly, causing you to do this, you may wind up with a deck full of undesirable cards, and a couple turns where you aren't able to do anything useful. This is particularly true with a card like Cellar; if you discard too many cards, all the cards you discarded will be shuffled back into your deck, and you might draw them all back again anyway.
On the other hand, frequent reshuffling can mean playing your important cards more often, so long as they end up in your discard pile in time. Deck discarders like Chancellor are specifically intended to expedite that; the sometimes opaque benefit of shuffling more often leads to many new players underestimating such cards, or even not understanding their purpose. However, the mere act of shuffling every turn does not necessarily indicate a good deck; an Advisor player may shuffle often, but never even get to play their important cards.
Duration cards (and some Reserve cards) are rather notorious for missing the reshuffle, which is often cited as a drawback against them. On the one hand, the next-turn effects of Durations are typically quite powerful, but if that Duration is played right before a reshuffle, it's going to be sitting in your discard pile until the next reshuffle, doing nothing.
A few cards can get around the "one play per shuffle" idiom, or even rescue Durations from missing the reshuffle. Some cards, like Treasury and Alchemist, can top-deck themselves; others, like Scheme and Scavenger, can top-deck other cards. Prince takes this to the extreme by simply playing a cheap Action every turn; there are also two Durations (Champion and Hireling) that stay in play for the rest of the game. All three of these cards don't care at all about reshuffles once they've been played. Also, any engine that can draw your entire deck every turn is similarly not impacted by shuffle control.
A few cards and card-shaped things have effects that depend on shuffling. Most of these (Stash, Star Chart, Order of Astrologers, Order of Masons, Avoid, Fated) allow you to manipulate the results of your shuffle, but Emissary just gives you an unrelated bonus if you shuffle while resolving it.
 Prior official rules (amended by Dominion Second edition)
- When his Deck is exhausted and the player needs to draw or reveal cards from his Deck, he shuffles his Discard pile to reform his Deck. He does not shuffle his Discard pile until he needs to reveal or draw a card from his Deck and cannot.
- At any point in the game, if a player has to draw or reveal more cards than are remaining in his Deck, he must draw or reveal as many as he can and then shuffle his face-up Discard pile to form a new face-down Deck. Then, he draws or reveals the remaining number of cards from his newly shuffled Deck.