This chapter deals with the observer effect in observation studies. It begins with an introduction to the observer effect as such, including factors that are presumed to have an effect on it. Additionally, suggested measures to counteract the observer effect in observation studies are presented. The main part of the chapter presents the results from a study that followed up on the observer effect in an observation study focused on medication communication and discharge, conducted at a hospital ward. These results are presented and discussed in relation to some of the factors that affect the observer effect. Lastly, recommendations regarding the observer effect, for planning, conducting, and following-up observation studies, are given.