Leaching is a critical operation in hydrometallurgy. The performance of the leaching sections of a processing plant usually has a significant impact on the performance of the plant as a whole. In addition, leaching reactors are usually complex, involving multiple reactions and involving gas, liquid and solid phases. In spite of the importance of leaching reactors, the understanding, and hence the design, of continuous leaching is based on concepts originating from batch leaching experiments. Many of these concepts, such as "rate-controlling step" and "psuedo-steady state" are inappropriately used to interpret the reactor performance or to design leaching reactors. In this regard, the leaching number is derived. The leaching number is defined as (t) over barr(s)/(l) over bar, where (t) over bar is the mean residence time, r(s) is the rate of shrinkage of the particles and (l) over bar is the mean particle size. In addition to the simplicity of the definition of this dimensionless leaching number, it is shown in this paper that it is the most important parameter in understanding the performance of a leaching reactor. (c) 2005 Elsevier Ltd. All rights reserved.