Item response theory (IRT) is an approach used for survey development, evaluation, and scoring. IRT models describe the relationship between a person's response to a survey question and his or her standing on a latent (i.e. unobservable) construct (e.g. math ability, depression severity, or fatigue level) being measured by multiple survey items. IRT modeling is used to (a) evaluate the psychometric properties of a survey, (b) test for measurement equivalence in responses to surveys administered across diverse populations, (c) link two or more surveys measuring similar domains on a common metric, and (d) develop tailored questionnaires that estimate a person's standing on a construct with the fewest number of questions. This entry discusses IRT model basics, the application of IRT to survey research, and obstacles ...