This chapter focuses on the impact of various substrates on the nutritional quality – in particular protein content - of the yellow mealworm larvae. The authors begin by describing the characteristics of rearing substrates for optimal yellow mealworm growth, including nutritional composition, biomass production, and feed conversion ratio. Next they analyse the yellow mealworm's protein content, and consider factors which may influence it. Finally the chapter touches on factors influencing the use of novel biomasses as substrates to increase the yellow mealworm proteins content, and discuss how the yellow mealworm has proven to be able to degrade polymer bio-plastics, thus opening new interesting opportunities to address the global plastic waste problem.