This thesis is pointing out and through the research explaining the differences between British and American English. The theme of this thesis was lexical differences between British and American English, but for a better understanding of the differences, we also explained grammar differences, spelling differences and differences in pronunciation. In the theoretical part of this thesis we wrote some historical differences between two varieties, we described the occurrence of American English, and we wrote some other most important differences between British and American English. The second part of this thesis is bringing results of research which was done in two grades of elementary school in Otoĉac and two grades of high school in Gospić. The goal of this research was to examine how much pupils understand lexical differences between British and American English and which variety they use more in everyday communication. The result of this research confirmed our hypothesis: high school pupils understand lexical differences between two varieties more than elementary school pupils; in both schools, pupils prefer American English in everyday communication.