We study the long-wave, modulational, stability of steady periodic solutions of the Kuramoto-Sivashinsky equation. The analysis is fully nonlinear at first, and can in principle be carried out to all orders in the small parameter, which is the ratio of the spatial period to a characteristic length of the envelope perturbations. In the linearized regime, we recover a high-order version of the results of Frisch, She, and Thual, which shows that the periodic waves are much more stable than previously expected. Papageorgiou, Demetrios T. and Papanicolaou, George C. and Smyrlis, Yiorgos S. Unspecified Center NAS1-19480; NAS1-18605; F49620-92-J-0023; NSF DMS-90-03227; NATO-CRG-920097; RTOP 505-90-52-01...