The purpose of this study is to discuss, in an orderly fashion, all of the known and recognized species of Aphelandra, so that botanists in the future may be able to identify their collections of the genus and detect further undescribed species. The genus was proposed in 1810 by Robert Brown to include three disjunct species of Justicia. The only comprehensive treatment of the genus appeared in 1847, when Nees von Esenbeck published a total of 47 species in 3 genera, two of which are in synonymy. As a result of the present study, in addition to the 31 newly described species, 167 taxa (165 species and 2 varieties) are considered as adequately describing the entities in this genus. The range of the genus extends from southern Mexico to northern Argentina and Brazil, being conspicuously absent in the West Indies. It is found at elevations between sea level and 4000 meters, in extremely local distribution in virgin forests. Aphelandra, one of the larger genera of the family Acanthaceae, is completely void of cystoliths, the familiar character by which most acanthaceous plants are recognized. Its flowering spikes are often large and beautifully colored, even to the bracts and bractlets, and in certain species variegated or colored leaves occur. Important characters in the genus that link large series of species are the presence or absence of spiny interpetiolar bracts; of teeth, spiny or otherwise, on the margins of the leaf blades or flower bracts; and of ocelli on the flower bracts. These plants are, as a rule, widely scattered and are often only sparingly floriferous. Intergrades between species are unknown, and species represented by more numerous collections exhibit very few pronounced variations in appearance or in essential characters. The history, distribution, ecology, morphology, cytology, anatomy, and taxonomy of the four species of Thrinax are presented. The objective is to demonstrate that quantitative characters, formerly believed to be important in distinguishing species, merely represent random selections from clinal patterns and are therefore of little value in distinguishing taxa. Certain characters described for the first timeleaf sheath, blade, color and puberulence of the inflorescencehave proved to be of great taxonomic value. Particular attention is given to the variability of Thrinax parviflora Sw. over a wide range of climatic conditions and to its unusual phenotypic behavior, in special situations, in the Cockpit Country and on the slopes of Mt. Diablo in Jamaica. The haploid chromosome number (determined at pollen-tube mitosis) of all four species is 18. Little difference in size or morphology of the chromosomes exists among these taxa. Anatomically all four species can be distinguished by comparison of leaf segment sections.