Laboratories are an important resource in scientific disciplines, enabling students to apply their knowledge and develop their skills. However, there are circumstances in which using a physical laboratory is not possible or not appropriate, for example when students are unable to come to a lab or when they need to engage with dangerous activities. In these cases, virtual laboratories provide a viable alternative. An online laboratory is an interactive environment for creating and conducting simulated science experiments. The lab could be accessed through the web, or as a program running on a computer, either in the classroom or at home. The aim is for a student to experience the procedures of carrying out a science experiment, including the consequences of making mistakes, and to get results. Online labs can also allow students to interact with real scientific equipment in ‘remote labs’. Despite concerns that some aspects of practical work such as the sights and smells of experiments in the physical laboratory are missing from the experience, virtual labs are becoming mainstream in higher education for science and engineering in many countries around the world.