The conidial fungi are those members of the Deuteromycetes which produce asexual, nonmotile and usually deciduous propagules (conidia) by de novo growth from, or conversion of a fertile hypha (conidiogenous cell). Accommodated in this group of fungi are most of the important pathogens causing plant and animal, including human, mycoses, many species associated with food spoilage and release of mycotoxins (e.g., aflatoxins), numerous microbes employed in antibiotic production and other industrial processes, several fungi which are potentially useful in biological control and a large number of microorganisms which have aroused the interest of experimentalists in many disciplines of biological sciences. The conidial fungi clearly rank as a very significant group within the Division Mycota. In spite of the recognized importance of conidial fungi, the taxonomy of this group is still confusing and cumbersome. Workers are often frustrated while attempting to identify species using conventional keys for identification which are usually based on highly variable morphological characters such as conidial shape, septation and pigmentation, conidiophore arrangement and cultural features. Investigations of conidial and conidiogenous cell development have provided new, genetically stable and thus, taxonomically reliable data which the systematist can incorporate into existing classifications in order to yield a more functional system. It has been this relationship between conidiogenesis and taxonomy that has been the principal impetus for continuation of these studies. Examination of aspects of conidium ontogeny, however, has recently involved additional research areas such as ultrastructure, cytology, biochemistry, plant pathology and medical mycology. The taxonomy of medically important conidial fungi is particularly chaotic. Many species are capable of alternating between yeast and hyphal phases both in tissue and pure culture, which increases the difficulty of accurate identification of these microorganisms. The minute size of most pathogenic species of such genera as Phialophora, Exophiala, Cladosporium, Sporothrix and Histoplasma also places strain on the mycologist to identify these fungi based on light-microscopic examinations of morphological characters alone. Electron-microscopic investigations of conidial fungi have provided new and taxonomically significant information by revealing aspects of conidiogenesis which have clarified earlier developmental concepts and defined new categories of conidium and conidiogenous cell ontogeny. By correlation of data obtained by light- and electron-microscopic analyses, certain key morphological features which are recognizable with the light microscope can be used, retrospectively, for identification of a particular mode of development. Such correlation is necessary for the formulation of a functional classification in which developmental characters are employed. The categories of conidial and conidiogenous cell development described in this paper provide the basis for incorporation of such characters into existing classifications of conidial fungi.