Any time that interviewers are used to collect survey data it is possible to leverage their observations to augment the self-reports solicited from respondents. While interviewer observations have been used in various forms for many years, it has not been until recently that this important form of paradata has been subjected to rigorous study. In some cases, interviewer observations may be very accurate and valuable, but in other cases they may be prone to serious errors that compromise their utility. This chapter identifies a few empirically validated uses of interviewer observations and discusses the best practices for these uses. The chapter also outlines several potentially important uses of interviewer observations where additional research is needed to determine the value and accuracy of the observations.