This chapter presents procedure for selecting inoculant vesicular-arbuscular mycorrhizal fungi based on an understanding of the criteria essential for their successful use. It is not possible to be prescriptive in this approach. Procedures for selection need to be designed for each situation. All of the techniques that are necessary for characterizing species of vesicular-arbuscular mycorrhizal fungi and for assessing mycorrhiza formation and functioning are described. Effectiveness, efficiency, efficacy, and effectivity are terms applied to species of vesicular-arbuscular mycorrhizal fungi. Species of vesicular-arbuscular mycorrhizal fungi are required that can either directly increase plant growth or indirectly increase plant growth by improving soil conditions. Direct benefits are usually related to the enhancement of phosphate uptake into the plant, but in some soils enhanced uptake of zinc, copper and ammonium may also be important. Processes that are important in enhancing phosphate uptake are: hyphal growth in soil outside the zone of depletion of phosphorus around roots; absorption of phosphorus by the hyphae; transport of phosphorus towards the root; and transfer of phosphorus to the root. The chapter also characterizes and compares fungi.