Until recently, with the exception of oral isotretinoin for the treatment of severe recalcitrant nodular acne, systemic therapy for acne vulgaris and rosacea has been based on anecdotal support, clinical experience, and small clinical trials. Tetracycline derivatives are the predominant systemic agents that have been used for both disease states, prescribed in dose ranges that produce antibiotic activity. Anti-inflammatory dose doxycycline, a controlled-release (CR) 40-mg capsule formulation of doxycycline that is devoid of antibiotic activity when administered once daily, was US Food and Drug Administration (FDA)-approved for the treatment of inflammatory lesions (papules and pustules) of rosacea, based on large-scale phase 3 pivotal trials and long-term microbiologic and safety data. Also, an extended-release (ER) tablet formulation of minocycline was approved by the FDA for the treatment of inflammatory lesions of moderate to severe acne vulgaris in patients 12 years and older based on large-scale phase 3 clinical trials that evaluated efficacy and safety, dose-response analysis, and long-term data. This article discusses the studies and clinical applications related to the use of these agents.