Adhesion to tooth structure, especially dentin, is one of the most attractive topics to researchers. Its role in daily practice and saving teeth made this topic under research spot. Great enhancement has been done to the adhesive systems throughout the last 30 years. Yet, many challenges still present and need further studies and solutions. Several studies have been done to enhance the performance of the adhesive system. Ultrasonic activation, electric current assistance and adding second coat of the bond. Studies result in different recommendation some advocate the second coat application and others were cautious due to many reasons as it might be misdiagnosed as caries beneath restorations or affect the mechanical properties of the adhesive layer. The chemical structure of the adhesive strongly affect its properties and its method of application. The all-in-one adhesives is more hydrophilic than other adhesive systems, which make the adhesive layer liable to degradation easily. Thus , double coat application may be useful. However, the two-step adhesive systems produce more hydrophobic adhesive layer, making the the double coat application questionable.