The authors report a series of original studies and analyze previous work in the area. "{\ldots} the position is taken that the frequency with which verbal units have been experienced is the fundamental variable responsible for the characteristics which have been used to define meaningfulness." The implications of the frequency hypothesis were tested in 16 experiments. These experiments deal with the effects of the frequency of letters and letter-combinations on serial and paired-associate learning, the effect of "pronunciability" on learning, the effect of frequency on letter-sequence habits, and the difference in the effect of the meaningfulness of a word depending on whether the word is a stimulus or a response.