Fungal mycelium, the vegetative structure of fungi, is emerging as a promising biological material for sustainable fabrication due to its biodegradability, low energy production requirements, and tuneable physical properties. Mycelium-based composites are engineered by growing fungal networks on various organic substrates, which can be moulded and processed into materials with diverse applications such as packaging, insulation, construction, and even bio-textiles. These materials are highly considered for their low environmental footprint and potential to replace synthetic, non-biodegradable products. This chapter provides a comprehensive overview of the cultivation of mycelium on various agricultural waste substrates, highlighting the eco-friendly nature of this approach. The ability to tune the physical properties of mycelium-based materials, including mechanical strength, density, thermal conductivity, and hydrophobicity, is crucial for their application in different industries. This chapter also highlights the primary methods used to modulate these properties, such as adjusting the type of fungal strain, substrate composition, growth conditions (temperature, humidity, nutrients), and post-processing techniques (e.g. drying, heat treatment, and chemical cross-linking).