This chapter discusses many types of collisions ranging from the collisions of subatomic particles to the collisions of galaxies. The molecular collisions are divided into intermolecular collisions and collisions of molecules with walls. The collisions of molecules are behind the sudden changes of molecular velocities in their absolute values and directions. With reducing the atomic mass of target atoms, the higher portion of the momentum carried by an incident particle is given to a target atom. The reduction of the kinematic factor is greater for the smaller mass ratio, which means that the projectile particle is backscattered with smaller kinetic energies. Since at free molecular conditions, the molecular collisions with walls are prevalent, they characterize behaviors of gas systems at transport phenomena and sorption processes. Thermal accommodation coefficients were introduced at interaction of gas molecules with solid surfaces by Knudsen and are related to the heat transfer phenomena by molecular conductivity via gaseous environments.