X-ray diffraction, dynamical mechanical analysis, and infrared reflectivity studies revealed an antiferrodistortive phase transition in EuTiO${}_3$ ceramics. Near 300 K, the perovskite structure changes from cubic $Pm\overline{3}m$ to tetragonal $I4/mcm$ due to antiphase tilting of oxygen octahedra along the $\mathbf{c}$ axis ($a^0{a}^0{c}^{-}$ in Glazer notation). The phase transition is analogous to SrTiO${}_3$. However, some ceramics as well as single crystals of EuTiO${}_3$ show different infrared reflectivity spectra bringing evidence of a different crystal structure. In such samples, electron diffraction revealed an incommensurate tetragonal structure with modulation wave vector q $\simeq$ 0.38 a ${}^{*}$. Extra phonons in samples with modulated structure are activated in the IR spectra due to folding of the Brillouin zone. We propose that defects such as Eu${}^{3+}$ and oxygen vacancies strongly influence the temperature of the phase transition to antiferrodistortive phase as well as the tendency to incommensurate modulation in EuTiO${}_3$.