Easily the most popular product in the brand's entire lineup, the Olaplex No.3 Hair Perfector ($28) is so effective that even a few of our own editors have sang its praises. It uses a special bond-building technology to repair damaged or colour-treated hair. Still, it shouldn't be used as a conditioner but rather as an intensive mask for anyone with severely damaged hair from chemical treatments. To get the best results, you should apply it to the hair at least twice a week before following it with your shampoo and conditioner.