  Information access systems, such as search engines, recommender systems, and
conversational assistants, have become integral to our daily lives as they help
us satisfy our information needs. However, evaluating the effectiveness of
these systems presents a long-standing and complex scientific challenge. This
challenge is rooted in the difficulty of assessing a system's overall
effectiveness in assisting users to complete tasks through interactive support,
and further exacerbated by the substantial variation in user behaviour and
preferences. To address this challenge, user simulation emerges as a promising
solution.
  This book focuses on providing a thorough understanding of user simulation
techniques designed specifically for evaluation purposes. We begin with a
background of information access system evaluation and explore the diverse
applications of user simulation. Subsequently, we systematically review the
major research progress in user simulation, covering both general frameworks
for designing user simulators, utilizing user simulation for evaluation, and
specific models and algorithms for simulating user interactions with search
engines, recommender systems, and conversational assistants. Realizing that
user simulation is an interdisciplinary research topic, whenever possible, we
attempt to establish connections with related fields, including machine
learning, dialogue systems, user modeling, and economics. We end the book with
a detailed discussion of important future research directions, many of which
extend beyond the evaluation of information access systems and are expected to
have broader impact on how to evaluate interactive intelligent systems in
general.
