The rapid depletion of natural resources and the environmental concerns associated with the use of fossil fuels as the main source of global energy is leading to an increased interest in alternative and renewable energy sources. Lignocellulosic biomass is the most abundant source of organic materials that can be utilized as an energy source. Anaerobic digestion has been proven to be an effective technology for converting organic material into energy products such as biogas. However, the nature of lignocellulosic materials hinders the ability of microorganisms in an anaerobic digestion process to degrade and convert organic material to biogas. Therefore, a pretreatment step is necessary to improve the degradability of lignocellulosic materials and achieve higher biogas yield. Several pretreatment methods have been studied over the past few years including physical, thermal, chemical and biological pretreatment. This paper reviews biological and thermal pretreatment as two main promising methods used to improve biogas production from lignocelluloses. A greater focus is given on enzymatic pretreatment which is one of the promising yet under-researched biological pretreatment method. The paper addresses challenges in degrading lignocellulosic materials and the current status of research to improve biogas yield from lignocelluloses through biological and thermal pretreatment.