This paper reviews the progress that has been made in understanding chemisorption from self-consistent electronic band structure calculations of adsorbate overlayers on transition metal thin films. Emphasis is placed on the information to be gleaned from the charge density difference associated with chemisorption. Simple interpretations are discussed in the following cases: saturation overlayers of hydrogen or cesium on tungsten{100}, of hydrogen or sulfur on nickel{100}, and of silver on palladium{100}. The basic features of LAPW method, used in many of these calculations, are reviewed.