Although different definitions exist, compliance can be described as the consistency and accuracy with which treatment regimes, laws, or guidelines are followed. The reasons for noncompliance might differ between owners of different animal species. However, the problem affects pets, horses, and livestock. Many compliance studies have been conducted in dogs, including those evaluating compliance with short-term antimicrobial treatment or long-term medication. Clinical studies often use different measures, such as owner self-reports, pill counts, or electronic monitoring caps, which results in different compliance rates. Retrospective studies used transaction data to approximate the compliance with long-term medication in dogs. However, noncompliance can also involve the following of management recommendations or guidelines, as shown in studies on horses and livestock. The presented examples highlight the extent of the problem in companion animals and livestock. Depending on the indication, noncompliance can substantially impact the outcome and should always be considered when evaluating health interventions in everyday veterinary practice.