  The following notes are based on lectures delivered at the research school
Modeling and Control of Open Quantum Systems (Mod\'{e}lisation et contr\^{o}le
des syst\`{e}mes quantiques ouverts) at CIRM, Marseille, 16-20 April, 2018, as
part of the Trimester \textit{Measurement and Control of Quantum Systems:
Theory and Experiments} organized at Institut Henri Poincar\'{e}, Paris,
France. The aim is to introduce quantum filtering to an audience with a
background in either quantum theory or classical filtering.
