The use of markers to determine digestibility of feeds overcomes the need to make exact measurements of feed intake and total faecal output in the traditional total collection method. Although several external and internal markers have been evaluated through the years, a marker that satisfies all the criteria of an ideal marker is yet to be found. This review describes the use of acid-insoluble ash (AIA) as a marker in digestibility studies. Three variations of the original gravimetrically method, based on burning of organic matter in the sample by ashing, boiling in hydrochloric acid, and re-ashing, are commonly used to determine AIA contents. A summary of the recovery rate of AIA determined in several species with different diets presented a mean around 100%. Of 45 studies where the AIA method was compared to the total collection method to determine digestibility of feeds in different species, 26 showed similar results, 9 an underestimation by the AIA method, and 10 an overestimation. No significant diurnal or daily variation in faecal AIA has been found in poultry, sheep, pigs, or cattle. Analytical error could be described as the most common reason for failure when using AIA as marker, especially in feeds with low natural AIA content. It is concluded that AIA presents a reliable marker with several advantages that could be successfully utilize to determine faecal digestibility in animal species under certain circumstances, and with the application of some precautions.