Biotechnology has emerged as one of the key environmentally safe technologies for the future which enables use of biomass to develop novel smart materials and to replace oil derived products. Fungi are the most efficient producers of the enzymes needed for this purpose and in addition they produce a plethora of secondary metabolites, among which novel antibiotics can be found. Industrial application and exploitation of the metabolic capacities of fungi requires highly productive and robust gene expression systems, which can be achieved by selection of appropriate species and strain improvement. In this book we aim to summarize homologous and heterologous gene expression systems of fungi for production of enzymes and secondary metabolites. A broad overview on requirements, challenges and successful applications shall serve as a basis for further development of fungi as biotechnological workhorses in research and industry.