One of the great challenges in next generation lithography is to print linear features with controllable sidewall roughness, which is usually called line edge/line width roughness (LER/LWR). The aim of this chapter is to provide an interdisciplinary approach to LER/LWR covering all related aspects. To this end, after a short introduction to LER/LWR concepts, it reports the basic findings of recent intensive research concerning the metrology and characterization, the material and process origins, and the device effects of LER/LWR. Both simulation and experimental results are presented, and emphasis is given to their comparison.