Arithmeticity of quaternionic modular forms on G_2 Abstract: Quaternionic modular forms (QMFs) on the split exceptional group G_2 are a special class of automorphic functions on this group, whose origin goes back to work of Gross-Wallach and Gan-Gross-Savin. While the group G_2 does not possess any holomorphic modular forms, the quaternionic modular forms seem to be able to be a good substitute.  In particular, QMFs on G_2 possess a semi-classical Fourier expansion and Fourier coefficients, just like holomorphic modular forms on Shimura varieties.  I will explain the proof that the cuspidal QMFs of even weight at least 6 admit an arithmetic structure: there is a basis of the space of all such cusp forms, for which every Fourier coefficient of every element of this basis lies in the cyclotomic extension of Q.