Over 18,000 frames of data acquired with a narrow-beam azimuth and elevation scan high-frequency backscatter sounder located at Boulder, Colorado, during the period October 1964 through June 1968 have been examined and classified. Analysis of these data has revealed that the ‘irregular structure’ of the midlatitude ionosphere is the rule rather than the exception. Irregularities of varying scale size and apparent motion were present in about 90% of the observations made during almost one-half a sunspot cycle. The ‘signatures’ observed by this HF radar system have been categorized into eight generic types, which have been labeled with names roughly describing their appearance on the range-azimuth scan record. The relative diurnal and seasonal occurrences, as well as the qualitative sunspot cycle and geomagnetic correlation of these signatures, are presented.