The main aim of this article is to identify those factors that affect treatment compliance, the extent to which the owners of patients comply with the treatment regimen, and how to improve compliance. The article is mainly based on reports from the medical literature. Although compliance can be measured in several ways, there is no valid and reliable method. For this reason, several methods should be used. Factors that affect treatment compliance in human medicine are (I) the expected effectiveness of treatment, (II) the severity of the disorder, (III) the risks associated with medicine use, (IV) the duration and dosage of medical therapy, (V) patient motivation, and (VI) the doctor-patient relationship. In the literature, in veterinary medicine treatment compliance is reported to range from 44% to 55%, whereas in human medicine it is reported to range from 5% to 96%. Compliance can be improved if (I) the owner can easily incorporate the treatment regimen in his/her daily life, (II) the treatment regimen does not disrupt the owner's daily routine, (III) the owner is informed about and understands the treatment provided, and (IV) the relationship between veterinarian and owner is good. In conclusion, many questions about treatment compliance in veterinary medicine remain unanswered, and research is needed to answer them.