This review describes the relationship between microstructure fabricated on the welding surface andwelding strength in laser welding of metal and resin. In the process of laser welding of metal and resin,welding surface of metal part was pretreated to fabricate the microstructure, such as microholes, and weldingsurfaces of metal and resin parts were locally heated by laser irradiation. Then, the locally melted resin flewinto the microstructure on the metal surface and strong joining could be formed. The influence of diameterand number density of microholes fabricated on the welding surface of metal part, as well as molding depth,on welding strength were investigated. It was found that the welding strength is affected by the area ratio ofmicroholes to the welding area, and reaches the peak at a certain value of the area ratio. It was also found thatwelding strength increases with increasing the molding depth and is saturated at a certain value of moldingdepth.

Plastic and metal injection molding joining technology has various benefits such as obtaining high joiningstrength, reduction of number of process, and realization of the hybrid structure with a complex structure.The joining technologies recently reported to scientific journals were classified into “intermolecularinteraction on metal and resin interface（ e.g. covalent, hydrogen bond）”,“ contact area on metal and resinjoint plane（ e.g. surface micro-structuring）”, and“ mechanical constraint on metal and resin interface（ e.g.anchor effect）. Their reports were reviewed focusing on the effects of metal surface treatments and injectionmolding conditions on their joining strength and their mechanisms. Recent analytical technologies with directin-situ observation and monitoring physical condition inside the mold during injection molding were reviewedand injection molding conditions to obtain higher joining strength were indicated.The establishment of the standard ISO19095 series for plastic and metal injection joining was mentionedand their usefulness were emphasized. Objectively reliable plastic and metal injection joining technologieswould be brought by ISO 19095. The results showing shear and tensile joining strength more than 40MPa andexcellent durability have been reported, which would be brought by the mechanical constraint mechanismon metal and resin interface（ e.g. anchor effect）. Advances in the analysis of joining interface during moldingand mechanical fracture are expected to spread metal and resin injection molding joining technology to theindustry more widely.

Recently, in order to reduce weight and cost of automobiles, smartphones, digital cameras, etc. as well asaircraft, the necessity of adhesion and bonding technology of many different materials such as metal, CFRP,ceramics, resin and the like has increased , and various bonding technologies have been developed.Conventional joining methods using adhesives require man-hours such as fixation of adherends and curingtime of the adhesive. Also, most of engineering plastics are crystalline and poor in adhesion. In order to solvethese problems, recently, a method of generating fine irregularities on the surface of the adherend by wetetching or laser treatment and directly injection-molding the resin thereon, a method of melting and bondingthe resin in the vicinity of the bonding interface with irradiating with laser beam, friction of the adherendmetal, ultrasonic waves, a hot plate or the like, and a method of chemically bonding the metal and the resintreated with the molecular adhesive by injection molding or pressurizing and heating, were developed. 　In these cases, the molten resin plays a role of an adhesive and does not require heating for curing as in thecase of adhesion, and the resin is solidified by cooling, so that there is an advantage that it can be joined in ashort time.In addition, as a chemical bonding method without using an adhesive, methods such as "coupling reactionmethod", "gas adsorption dissimilar material bonding method", and "water vapor-vacuum ultraviolet bondingmethod" were developed. The main joining methods of these are classified according to the joining principleand outlined.