Even though considerable advances have been made in the fields of boiler design and coal characterization, ash deposition on heat transfer surfaces continues to be a significant problem in existing conventional utility boilers. A cost effective way to deal with this difficulty is the continuous monitoring of fouling tendencies. These techaniques have become a widespread practice in coal-fired power stations as a tool for operation optimization. In spite of that, little information has been given about design criteria and case-study experiences. In this paper, the fundamentals, concepts and application principles are reviewed, attempting a systematic analysis of current developments and open questions.