The first module in the Sclerotherapy series begins with a review of the diagnostic assessment and modern principles of venous treatment using sclerosants. Various presentations of venous disease are discussed with a focus on determining the source of the reflux and how to obtain the best possible patient outcome. Following a review of contraindications, treatment options for telangiectasia, reticular veins, and varicose veins are considered. The mechanism of action for common sclerosants is presented followed by a discussion of post-procedure treatment, charting, and documentation of results.