One of the most troublesome groups of the Boletaceae, as far as the recognition of taxa is concerned, has been the group generally known as the section Viscipelles of Boletus in the older literature. This group represented a "gray area" between Boletinus on the one hand and Boletus sensu lato on the other. However, attempts to distinguish species in Suillus were frustrating not so much because of the difficulty of recognizing them in nature by the ordinary characters used to distinguish species and varieties in the fleshy fungi generally, but because of the difficulty of interpreting the conflicting statements in the literature or one's inability to find specimens whose characters compared well with those given in the published descriptions. There is no denying that the species of Suillus are one of the most confused groups of fleshy fungi in North America. We offer here our observations and interpretations in the hope that future students will find them useful and to the point on taxonomic problems. Our treatment does not introduce much in the way of new characters, but we hope we have made a contribution toward a better evaluation of the features emphasized by previous authors, as well as having standardized others, such as the color of the spore deposit, so that their interpretation by future workers will be more accurately and readily made.