When properly designed, pretreatment may enhance the methane potential and/or anaerobic digestion rate, allowing better digester performance. The purpose of this paper is to give guidelines and present a rationale on pretreatment techniques of the main types of waste used in biogas plants. The pretreatment strategies proposed are supported by a literature review and the experience of three research groups working on this topic: INRA-LBE, UPC and FORTH. Thermal pretreatment should be recommended for waste activated sludge as it increases its methane potential and digestion rate, it allows sludge sanitation and the necessary heat energy is produced on-site. In the case of microalgae, thermal pretreatment has shown to be the most effective in terms of biomass solubilization and methane yield improvement. As for sewage sludge, the heat needed is produced on-site, although depending on pretreatment conditions biomass thickening or dewatering could be required. Saponification is a pretreatment which is preferred to solubilize fatty residues and to increase their bioavailability in the digesters. For fatty residues originating from slaughterhouses, classified as category II waste, this pretreatment can be optimized to ensure both sterilization and solubilization in the same process. For lignocellulosic biomass, the first aim of pretreatment should be delignification, for which alkali pretreatments are effective.