The electronic basis to the surface chemical bond of molecules and atoms chemisorbed to transition-metal surfaces is introduced. The chemical bonding features that determine preference of an adsorbate for different coordination sites are identified. This is related to a discussion of chemisorption as a function of particle size. Lateral effects relevant at high surface coverage are discussed with chemisorption-induced surface reconstruction. The analysis focuses on the relation of the surface chemical bond energy with degree of delocalization of surface transition-metal d-valence electrons and distribution of electrons over bonding and antibonding adsorbate complex fragment orbitals. The model of chemisorption as surface molecule complex formation embedded into the surface of a metal is shown to be a good approximation. This chapter concludes with an analysis of transition states of elementary surface reactions of small adsorbed molecular species. The structure and energy of such transition states are shown to relate in an interesting way to the nature of the chemical bond to be activated (p-bond vs s bond) and the topology of the reaction center.