Derma rollers is a great device that is used for so many skin issues such as scars, acne, stretchmarks, bald hair, black spots, uneven skin etc. The length you use depends on the skin condition being treated. 0.5 for hair growth, anti-aging, skin whitening, acne and dark spots. 2.0 for scars and Stretch marks. It is presently the only other treatment that works for scars besides surgery and laser. WARNING there are cheap plastic dangerous derma rollers being sold all over, always be careful and ensure you buy high quality titanium grade, just like the ones we sell. We are people who actually use our products.