AbstractThe Complexity of the behavior of smoke at fires can lead to great difficulties in obtaining measurements from instruments which are both reliable and correlatable with human observations. The first article in this two‐part review considers the principal characteristics of smoke, the factors which influence its behaviour at fires, the basic theory of light transmission through smoke, and the behaviour of the eye in smoky conditions. This information will be used in the second article to assess the performance and limitations of various types of smoke measuring instrument