Biomass co-firing is recognised as a crucial technology to aid in curbing the use of fossil fuels, particularly due to its relative ease of implementation. This article provides an introduction to bicimass and its use as a fuel - as with any fuel, this includes its characterisation and energy conversion. Key fuel properties, both chemical and thermo-chemical, are described. Combustion is the energy conversion technique that is focused on; the implications of biomass co-firing are discussed along with an overview of current co-firing technology. Biomass pre-treatment techniques are identified as a means of alleviating some of the drawbacks of co-firing, and this is the chief focus of this paper. A comprehensive review is carried out on torrefaction and leaching, which aim to enhance the physical/thermochemical and chemical properties of the biomass, resptctively. Milling and pelletising are also looked at. Additionally, the practical aspects of implementing biomass pre-treatment are explored, and areas where further research work is needed are identified. The final section of the review is concerned with CO2 avoidance, which is one of the key drivers behind adopting biomass co-firing. (C) 2017 Elsevier B.V. All rights reserved.