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 techniques 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. The main operational measure related to ash fouling is on-load cleaning by mechanical means. Based on a fouling monitoring system, improvement of sootblowing procedures can lead to significant savings, although a rigorous approach is not a trivial issue. The paper reviews the state of the art, as well as several stages in cleaning optimization. Finally, an attempt is made to formulate possible perspectives.