According to the World Health Organization, public health surveillance is a continuous process of collecting, analyzing, and interpreting health data such that the results can be directly disseminated to responsible actors to take immediate action if necessary. Although disease surveillance has its roots in managing infectious diseases, it has also become essential for tracking non-communicable diseases (NCDs) as well as health-related behaviors and contextual factors. Due to the increasing digitalization of the health space, traditional methods of data collection are being complemented by digital information such as Electronic Health Records or self-generated internet data, e.g., on social media. In this chapter, we will first define the term surveillance before introducing different approaches that are used depending on whether the focus is on infectious diseases or NCDs. Here, we will distinguish two major threads that underline the huge impact of digitalization on surveillance systems, i.e., the usage of digital technologies for primary data collection and the usage of routinely collected available digital data. We will then provide some prominent examples of surveillance systems before we conclude by highlighting the importance of the FAIR guiding principles for data sharing and of record linkage for disease surveillance.