For phase transitions and fluid dynamics, there are significant qualitative differences between two dimensions and three dimensions. However, it has been long assumed that the glass transition is similar in two and three dimensions. Here, we present experimental data on 2D and 3D colloidal systems near their glass transitions. We demonstrate that the differences between two dimensions and three dimensions are due to long-wavelength fluctuations, precisely those that distinguish 2D and 3D phase transitions. We show that the influence of these fluctuations can be disentangled from the underlying dynamics, and that 2D and 3D glass transitions are otherwise similar.