Filarial worms are a unique group of parasites with importance in both human and veterinary medicine. These parasites are typically long‐lived and difficult to detect, often causing chronic disease states over a period of years and, for these reasons, effective diagnostic testing is crucial for their control. Adult filarial worms tend to occupy inaccessible anatomical sites within the host, but microfilariae disperse widely in the blood or skin to allow uptake and transmission by the hematophagous insects necessary to complete the life cycle, and the detection of this microscopic stage represents a fundamental form of diagnostic testing. Immunodiagnostic and DNA‐based tests have since been developed for several filarial species, as well as methods for visualizing adult parasites in situ . All these techniques carry their own distinct strengths and weaknesses, so reliable diagnosis often requires a strategic combination of tests. Accurate diagnosis is important for potentially fatal infections like canine heartworm and is also essential for identifying emergent zoonoses, like Onchocerca lupi , and potential animal reservoirs, as with Brugia malayi . Accurate parasite detection and identification is useful not only in clinical settings but also greatly assists research efforts. This chapter will review the diagnostic methods available for some of the most common species of filarial nematodes in small animal veterinary medicine.