This presentation will highlight, for nanoelectronic device examples, how the effective mass approximation breaks down and why the quantum mechanical nature of the atomically resolved material needs to be included in the device modeling. Atomistic bandstructure effects in resonant tunneling diodes, ultra-scales Si slabs, Si nanowires, and alloyed quantum dots will be demonstrated in intuitive pictures. The presentation concludes with a brief overview of the empirical tight-binding method that bridges the gap between material science, physics, and electrical engineering for the quantitative design and analysis of nanoelectronic devices.