The paper is a brief overview of data obtained by A.A. Baikov Metallurgical Institute of the Russian Academy of Science in the field of bonding mechanisms for thermal spray processes. The following 3 phases of bonding are considered: forming of a physical contact between a particle and a substrate; activation of the contacted surfaces and possible chemical interaction between a spraying particle and a substrate; diffusion. Thermal and mechanical activation of a substrate are considered in the paper. It is shown that thermal activation by molten particles and substrate preheating have a major input into the bonding process of the thermal spray processes such as APS, twin wire arc and flame spraying with relatively low velocity of molten particles. Application of high velocity processes like HVOF, HVAF and Cold Spray changes the mechanism of bonding activation. In this case the deformation of a substrate in the contact zone and related bonding activation caused by dislocations play a major role in the bonding of a particle with a substrate. It is shown that the energy needed for bonding depends mainly on the properties of the substrate and the particle materials.