11. A computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a content item request, the content item request specifying a phrase with which content items are to be selected; determining that two or more targeting keywords for a same content distribution campaign are matched by the phrase, the determination including determining that a first kind of match between a first targeting keyword and the phrase is different than a second kind of match between a second targeting keyword and the phrase, the first kind of match being a more specific match than the second kind of match; determining which of a first targeting keyword and a second targeting keyword is associated with a higher bid; selecting, based on the determination of which targeting keyword is associated with a higher bid, a tiebreaker rule with which a controlling keyword will be selected from the two or more targeting keywords, wherein: a first tiebreaker rule is selected in response to the determination that the first targeting keyword is associated with the higher bid, the first tiebreaker rule specifying, at least in part, that the second targeting keyword is selected as the controlling keyword when the second targeting keyword has a higher quality score than the first keyword; and a second tiebreaker rule is selected in response to the determination that the second targeting keyword is associated with the higher bid, the second tiebreaker rule specifying that selection of the controlling keyword is based, at least in part, on an augmented bid for the second targeting keyword, the augmented bid for the second targeting keyword being lower than the bid for the second targeting keyword; selecting, based on the selected tiebreaker rule, one of the first targeting keyword and the second targeting keyword as a controlling keyword for the content item request; and providing data associated with the controlling keyword.
11. A computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a content item request, the content item request specifying a phrase with which content items are to be selected; determining that two or more targeting keywords for a same content distribution campaign are matched by the phrase, the determination including determining that a first kind of match between a first targeting keyword and the phrase is different than a second kind of match between a second targeting keyword and the phrase, the first kind of match being a more specific match than the second kind of match; determining which of a first targeting keyword and a second targeting keyword is associated with a higher bid; selecting, based on the determination of which targeting keyword is associated with a higher bid, a tiebreaker rule with which a controlling keyword will be selected from the two or more targeting keywords, wherein: a first tiebreaker rule is selected in response to the determination that the first targeting keyword is associated with the higher bid, the first tiebreaker rule specifying, at least in part, that the second targeting keyword is selected as the controlling keyword when the second targeting keyword has a higher quality score than the first keyword; and a second tiebreaker rule is selected in response to the determination that the second targeting keyword is associated with the higher bid, the second tiebreaker rule specifying that selection of the controlling keyword is based, at least in part, on an augmented bid for the second targeting keyword, the augmented bid for the second targeting keyword being lower than the bid for the second targeting keyword; selecting, based on the selected tiebreaker rule, one of the first targeting keyword and the second targeting keyword as a controlling keyword for the content item request; and providing data associated with the controlling keyword. 29. The system of claim 11 , wherein selecting one of the first targeting keyword and the second targeting keyword comprises: in response to selecting the first tiebreaker rule, ranking the first targeting keyword and the second targeting keyword based at least in part on a selection score associated with each of the first targeting keyword and the second targeting keyword, the selection score for each targeting keyword being based at least in part on the bid for the targeting keyword; wherein selecting one of the first targeting keyword and the second targeting keyword as the controlling keyword comprises selecting a highest ranking keyword as the controlling keyword.