For a half century or longer, the general conception of Pennsylvanian climate has been that of a mild, tropical, or almost tropical, humid condition of world-wide extent. Although this conception results in part from the Laplacian hypothesis, it results mainly from the interpretation of certain characters of the floras and faunas of the period. These characters are summarized as follows: large size of Pennsylvanian insects, coral reefs in Spitzbergen, climacteric development and wide distribution of benthonic Foraminifera, cosmopolitanism of floras and faunas, many structural features of the plants, and stagnation of evolution. A survey of the recent literature on the possible climatic implications of these characters does not support the contention that they imply the ki d of climate that they have been represented as postulating. Although some of the faunal and floral characters apparently have no climatic implication whatever, other characters cited as evidence of warm climate imply the reverse. Some characters of supposed climatic significance indicate merely environmental conditions, such as local water temperatures, warm currents, or swampy surroundings, having little or no climatic significance. Certain characters preserved in organisms from the Pennsylvanian to the present are commonly interpreted as having climatic significance because the contemporary organisms with these characters are adapted to certain climatic conditions. However, this interpretation disregards entirely the possibility of adaptations during the vast span of time subsequent t the Pennsylvanian.