Big data is characterised not only by the size of data files, but also by the diversity of data sources and continuity in data collection. Technological developments make it possible to store and analyse ever larger and more complex data files. Unlike more conventional research data, big data are often collected without explicit research questions in mind, and analytical techniques are used to find patterns in the data or to generate hypotheses. Examples of big data in medical research are the large data files of routinely collected data on care, sometimes enhanced by information from other sources such as mobile telephones. Using big data in research can be a good way to generate hypotheses or to answer specific research questions, but the use of big data should not be an end in itself.