Keratoconus is a condition resulting in corneal ectasia. Abnormal protrusion of the cornea leads to reduced visual acuity. Riboflavin induced collagen cross-linking (CXL) has received significant attention the last few years and has shown to stop the progression of keratoconus. The original CXL procedure involves epithelial debridement, application of topical riboflavin drops and UVA exposure at 370 nm for approximately 30 minutes. However, not all patients are suitable for this treatment. Collagen cross-linking is a relatively new method and several complications of the treatment have been reported. The treatment may greatly reduce the need for corneal transplantations. This article discusses CXL. Current studies show that the stiffening effect of CXL appears to remain stable after 6 years. However, the exact duration of the corneal stiffening effect is as yet unknown, and more prospective randomized controlled trials in the future are desirable.