Surveillance has been and continues to be essential to effective prevention and control of viral diseases. However, given current global challenges, surveillance efforts must urgently modernize to maximize their utility. This chapter describes the history and principles of surveillance for viral diseases with an emphasis on the importance of key epidemiological concepts such as temporal fluctuation and bias. All surveillance processes, including system development, data collection and analysis, evaluation, and communication, should be directly aligned with the purpose and local context. The strengths and limitations of using various data sources for surveillance are discussed and linked to common analytic strategies. Finally, a framework based on lessons learned from past outbreaks is applied to consider the future of viral disease surveillance.