"If all the AHAs on the market were Kardashian sisters, glycolic acid would be Kim. The ingredient has the most research behind it and the longest history of proven results for different skin types and ages—it's undoubtedly the star of the AHA family. Glycolic acid can come from sugarcane, but because the synthetic form is easier to stabilize, that's what you'll find in most skincare. A small molecule, it penetrates the skin really well for maximum cell turnover and also encourages the skin to naturally hydrate itself. You can use glycolic acid alone or in combination with other AHAs, and it's available over-the-counter in a number of different concentrations, starting at 5% and going up to around 40%." -Byrdie