The aim of this thesis was to study change in the meaning of the word cool, comparing the 1890s, the 1950s and the 2010s. Examples where the word cool was used were obtained from the Corpus of Historical American English, which is created by Mark Davies, and analyzed in terms of in which senses and text types they were used. The results show that the biggest meaning change in cool between the 1890s and the 2010s is that the most prominent sense in the 1890s is ‘calm’, and ‘slightly cold in a pleasant way’ not far after, and in the 2010s the most prominent sense was ‘pleasant, enjoyable or likable’. The most frequent senses of cool in the 1950s were ‘slightly cold in a pleasant way’, ‘temperature that is too cold’, and ‘calm’ as the most commonly used. The most prominent text type where the word cool occurred was fiction in all three analyzed decades.