Selecting thermoplastics for welded, under-the-hood, automotive parts strongly depends on the plastic material properties, part design, as well as the molding and joining / welding technology conditions. Laser transmission welding (LTW) requires preferential deposition of energy and subsequent melting and diffusion of the material in the interfacial / weld-plane zone. This is optimized when the laser beam is transmitted through the thermoplastic transparent part and absorbed by the adjoining part to be welded. Energy deposition can be controlled to some extent by adjusting laser power, choice of beam focussing optics, sweep rate, etc.The thermoplastic material properties and composition (reinforcements, fillers, additives, pigments, etc.) may have the greater influence and need to be characterized for optimum material selection for the transmission welding application. Commercial nylon based thermoplastics cover a large array of compositions, which may affect the laser transmission welding (LTW) process.To provide a guide to nylon based thermoplastics selection for these applications we have evaluated the influence of specific material composition factors and properties, such as fiber-glass, mineral filler, impact modifier content, and color / pigment version on the Near InfraRed (NIR) transmission characteristics, including the laser wavelength (1.06 mm). We have related these findings to the mechanical performance of nylon welded joints in terms of the influence of transmission laser welding technology parameters (laser power, welding speed, laser beam spot diameter, clamp pressure, etc.) and thermoplastic composition (reinforcements, fillers, additives, pigments, etc.).Comprehensive results of this evaluation will assist plastic parts designers and technologists in selecting nylon based thermoplastics and developing new products using laser transmission welding (LTW) technology.