Scaling is the act of using an electronic scale to determine whether or not a particular pack has a high-rarity card in it. This is possible because a sensitive scale can pick up the minute differences between the weights of packs containing a Super Rare or Ultra Rare card, as such cards are slightly heavier than a regular Rare or Common card. Scalers, however, have trouble picking up Ultimate Rares, because the weight of the foil technology of that rarity is very, very close to that of a Common.
Many have argued that Yu-Gi-Oh! is becoming increasingly expensive, and the likelihood of getting the rarest cards further reinforces that. Subsequently, Scaling is becoming popular amongst the low-income crowd, who seek to further their collection while spending what they deem to be a "reasonable" amount. It is not a criminal offence to scale in either the United Kingdom or the United States, because it is not tampering with the goods. Some retailers, however, operate a strict "no scaling" policy on Booster Packs, by storing them behind a counter.
Scaling can be seen as fundamentally unfair to other people, who then buy booster packs out of a box that has been "scaled". In this situation, not only are there a greatly reduced number of packs, but it is increasingly unlikely that anything more significant than a Rare will be found. Upper Deck Entertainment repeatedly stated its opposition to scaling, as scaling is unfair to players who don't do it, and it resulted in a disproportionate profit, both to Upper Deck, and to retailers.