Biomass provides 14% of the world’s primary energy production, but it is largely wasted by inefficient and unsustainable use. To exploit the full potential of this energy source, new approaches and modern technologies such as pyrolysis and gasification are needed. Pyrolysis is the most promising thermal decomposition method for the conversion of biomass into valuable bio-products. The process produces a solid fraction (biochar), a liquid fraction (bio-oil) and a mixture of gases. Depending on pyrolysis conditions, biochar for soil amendment, activated carbon, carbon fibers, bio-fuels, value-added chemicals (PF type adhesives, phenolics, levoglucosan, octane enhancers, fertilizers) and gas products (hydrogen, methane, ethane and propane) could be achieved. The ratio of the products varies with the chemical composition of the biomass and operating conditions such as pyrolysis temperature, heating rate, reactor configuration, pyrolysis atmosphere, reaction time, particle size and so on. In the scope of this issue, this chapter covers the definition and sources of biomass, thermal behavior of biomass and its components, fundamentals of the pyrolysis process, and effects of the process parameters on yields and composition of products. Moreover, properties of bio-oil and biochar are explained according to their utilization areas.