National veterinary services emerged in response to the animal disease challenges of centuries past. Their often singular preoccupation with exogenous disease preclusion yielded, by the early 1900s, to parallel commitments to control a number of serious indigenous, zoonotic infections. Advances in clinical pathology and epidemiology opened the door in many countries to disease, if not agent, eradication initiatives by the latter part of that century. The attending interval witnessed the explosive growth of international trade and the emerging or re-emerging conditions that threaten to accompany it. Animal disease agents, their hosts and the environments within which they interact vary considerably from nation to nation. Depicted as a medieval metaphor, the exogenous cluster around the walls of the castle, awaiting entry upon the next uninspected, unsuspecting fomite or beast. Some of these threats reside comfortably within the structure's bedding and detritus, sapping the vitality of its residents until detected and exterminated. Others emerge from the least expected crevices, growing insidiously or explosively in tandem with changes wrought by humans or nature. In anticipation of, or in response to, these myriad challenges, national veterinary services mount surveillance campaigns. From the national survey that denies the presence of exogenous disease to the post-eradication assessment that confirms its demise, surveillance forms the sensory acumen of the service. From the passive assessments that detect the emerging to the active regimens that plot progress against the indigenous, they form a continuum that defines the very nature of the national veterinary service.