**1. Introduction**

Non-edible lignocellulosic biomass materials are attracting increasing attention as renewable, economical, and abundant resources to reduce dependency on petroleum resources and minimize energy and material feedstock costs. These resources do not cause additional increase in the carbon dioxide level in the earth's atmosphere compared to fossil-based energy fuels such as coal, gasoline or natural gas. The carbon dioxide captured in biomass growth mostly balances with the release of carbon dioxide from bioenergy/biofuel. Therefore, use of biomass energy has the potential to reduce greenhouse gas emissions.

Biomass materials are the largest carbon sources for production of various fuels, chemicals and platform compounds and bioproducts. Because of their heterogeneous, complex and rigid structures it is hard to breakdown these materials to smaller components and/or convert to a wide range of value-added products. Biomass has a relatively low energy density; therefore, it requires more biomass feedstocks to supply the same amount of energy as a traditional hydrocarbon fuel. High oxygen contents of biomass materials can also negatively affect their conversion to various products such as fuels. For instance, in order to produce hydrocarbons fuels that can be comparable with petroleum-based ones oxygen should be removed from biomass structure. Efficiency of conversion processes can also varied depending on the biomass types (hardwood, softwood, grass, etc.).

Following sections will focus on the challenges for conversion of biomass to biofuels in detail.
