The demand for biomass for the production of energy in the forms of electric power, heat, and liquid fuels is growing fast over the last few years worldwide, mainly due to environmental concerns regarding the use of fossil fuels, such as coal, and oil. Agricultural residues, such as wheat straw, olive residue, and corn stover, as well as energy crops, e.g., Arundo donax, miscanthus, and switchgrass gain more and more interest as feedstocks to supply the vast amounts of biomass needed in order to produce the energy and liquid fuels required to fulfill the Kyoto protocol and reduce the environmental burdens of using fossil fuels in our planet. Biomass feedstocks can be converted to energy vectors in high-efficiency thermochemical systems, such as pressurized fluidized bed boilers and gasifiers coupled with Fischer–Tropsch or other types of liquid fuel synthesis units, supercritical combustion units, and entrained flow combustors or gasifiers. This use is presently limited by the ash-related problems associated with high-temperature/high-pressure environments. Inorganic bioresource constituents, such as alkali metals, chlorine, sulfur, and phosphorus, are the most troublesome elements responsible for several problems occurring. This paper presents a critical discussion regarding the main factors affecting the use of biomass of both agricultural residues and energy crops in high temperatures and high-efficiency thermochemical conversion systems for the production of energy and liquid fuels. On the basis of an analysis of the particular problems, we identify solutions that could resolve these problems and assist significantly with a much higher production of energy and fuels from various biomass sources in an economical and sustainable way.