Bioenergy is a critical component of the global transition to a sustainable energy system, offering versatile pathways to produce renewable heat, power, and fuels from organic materials. This paper provides a critical review of biomass diversity and bioenergy conversion technologies including thermochemical and biochemical conversion pathways. Mature pathways, including anaerobic digestion and co-firing in existing power plants, are commercially deployed (TRL 8-9) and offer immediate decarbonisation opportunities. Thermochemical technologies such as pyrolysis and gasification provide flexible platforms for producing bio-oil and syngas, but their widespread adoption is hindered by challenges related to product quality, including bio-oil instability and tar formation. For second-generation biofuels, hydrolysis remains the key enabling step, though the high cost of enzymes presents a significant economic barrier. Holistic sustainability assessment, supported by Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA), shows that although bioenergy has substantial greenhouse gas mitigation potential, its sustainability is highly dependent on land-use management to prevent competition with food production, while also highlighting the role of bioenergy with carbon capture and storage (BECCS) as a viable negative emissions technology. Future developments are focused on advanced biofuels like Sustainable Aviation Fuel (SAF) for hard-to-decarbonise sectors, the integration of Artificial Intelligence (AI) for process optimisation, and the evolution towards integrated biorefineries that maximise value within a circular economy framework.