Solid materials are commonly annealed to enhance their characteristics. Mono-crystalline and polycrystalline conductors and semi-conductors are normally annealed at different temperatures and under different atmospheres in order to enhance their surface morphology, to increase grain contact, to remove surface defects and to increase charge carrier mobility. During annealing a given substance, more imperfections occur as more bonds are broken and more atoms emigrate from stable positions to metastable positions, until a new equilibrium is reached. This follows the logic of the Arrhenius equations. Upon cooling, the atoms in the meta-stable positions return back to their stable positions and the solid becomes more crystalline. Fortunately, imperfections created during crystal growth, or caused by other effects, are cured during the return back of other meta-stable atoms. However, curing imperfect crystals by annealing should be considered with extra care. In many cases, slow cooling is needed to give enough time for the meta-stable atoms to pack inside their stable positions, while quenching causes permanent imperfections in the crystal. This is well known for different metals and to a lesser extent for semiconductors with relatively wide band gaps as we have recently observed. In cases of narrow band gap semiconductors, slow annealing may not be recommended as more imperfections would occur even during cooling, i.e. longer annealing time causes longer exposure time for the heated materials. Another issue to care about is the annealing temperature itself. It is reported that semiconductors with narrow band gaps are thermally unstable and should be carefully annealed at low temperatures for short time only. In this plenary, we will give an account on our latest finding on how annealing temperature and cooling rate may affect different mono-crystalline and polycrystalline semiconducting materials. Effects on crystal surface morphology, particle sizes, charge carrier mobility, and photovoltaic (PV) properties will be described. Effect on PV characteristics of multilayered metal chalcogenides semiconductors will also be addressed. A model to explain such effects will be presented.