Security Labels provide an important mechanism for controlling access to information in many high-security environments, and are also useful in environments with lower security requirements. This paper provides a reasonably detailed description of how security labels and clearances work, while attempting to avoid the high level of technical complexity seen in many papers in this area. This paper starts by looking at how security labelling is used for paper documents and other non-electronic applications. Then it looks at how Security Labels work for electronic documents and other online applications.