Population censuses, registers, and surveys are the primary sources of demographic data, including information about the size, composition, and characteristics of a population or population subgroups. A census is an enumeration of all households in a well-defined territory at a given point in time. Population registers are systems of continuous registration, maintained by certain countries, to keep demographic records of individuals. Surveys are used to collect detailed data on a particular subject from a sample or subset of the population. All three sources of data – censuses, registers, and surveys – are often used to monitor changes in population size and composition. In recent years, technological advances have led to a rapid increase in the availability of demographic data from unofficial sources, especially through online social networks and other Web-based applications. These “big data” sources provide many new opportunities for research and marketing, but also present challenges in terms of accessing the information, using it effectively, and protecting confidentiality.