**1. Introduction**

This chapter deals with the worldwide and local resources of challenging (or low-grade) biomass feedstocks available for the production of energy and chemicals. It aims to highlight the potential of different sources of low-grade biomass and make aware of their content of impurities, mainly ash-forming elements (e.g., Na, K, Ca, Si, P, S, and Cl) and nitrogen (N). It also suggests what kind of utilization techniques, such as combustion, pyrolysis, and gasification render the best yields and the least problems in industrial conversion systems.
