Abstract According to recent studies, about 60% of plastics failures come from a wrong selection of the used plastic grade. This chapter aims at helping to define the problem, pointing out the difficulty of a right choice of a defined plastic material taking into account a broad range of interactive parameters linked to the polymer properties, on the one hand, and surrounding stresses and media, on the other hand. Some thermoplastic features can surprise designers usually working with steels and other metals: the sensitivity to temperature, humidity, creep, chemicals, and viscoelasticity. The final choice results from numerous iterations, leading to a subtle balance of technical requirements, economic considerations, and targeted lifetime. The ins and outs linked to the main interactions between thermoplastics properties (mechanical, physical, aging, sensorial), surrounding parameters (temperature, time, stresses, chemicals…), economics, regulations, green trends, and lifetimes are examined. A checklist of 125 good questions concludes this chapter.