Enzymatic cycling provides a methodology for virtually unlimited amplification of analytical sensitivity. The most widely applicable cycling systems are those for NAD and NADP, since these can be used to increase the sensitivity of methods for a host of other substances. However, cycling systems for ATP plus ADP, GTP + GDP, glutathione and coenzyme A have also proven to be very useful. A total of 19 cycling procedures are described in greater or lesser detail. Some of these are capable of amplification rates in excess of 20,000 per hour in a single cycling step (20,000 x 20,000 with two one hour cycling steps). Advantages, disadvantages, limitations and other practical considerations are stressed, as well as the means for coupling the cycling systems to assays for other substances.