Abstract: The task difficulty is one of the important factors that affect the user's search behavior. This research systematically investigates and analyzes the researches related to task difficulty in the field of interactive information retrieval. It discusses from the definition, measurement of task difficulty, user perception reasons, the relationship between task difficulty and search behavior, and task difficulty prediction. Based on those findings, it puts forward further directions in future study.

Key words: Task difficulty, Information search behavior, Task difficulty prediction, Interactive information retrieval, User behavior, User experience