This guidance provides recommendations for clinical trials designed to assess the adhesion performance of transdermal and topical delivery systems (collectively referred to as TDS). Adhesion performance is defined in this guidance as whether the TDS fully adheres to the subject in the applied location for the duration of use of the TDS. Adhesion performance can affect both safety and effectiveness of TDS products because adhesion failures can result in reduced effectiveness caused by suboptimal dosing or potentially increased exposure when a new TDS needs to be applied sooner than the scheduled dose. Additionally, partial or full detachment of a TDS from a patient’s skin may result in unintentional exposure of the active pharmaceutical ingredient to a partner, child, or other individual, potentially exposing them to the drug’s toxicity. Adhesion performance may also inform the Dosage and Administration section of labeling.