Many Penicillium species produce enzyme systems with good performances in lignocellulose degradation. In our laboratory, lignocellulolytic enzyme-producing Penicillium oxalicum (formerly classified as Penicillium decumbens) strains have been studied for more than 30 years. High cellulase-producing mutants have been obtained through random mutagenesis and genetic engineering, and the components in the enzyme systems have been elucidated using systems biology tools. The effects of different carbon sources on the production level of lignocellulolytic enzymes have been studied, and the related molecular mechanisms have been investigated. When compared with the widely used cellulase producer Trichoderma reesei, some unique features have been found in P. oxalicum, including higher β-glucosidase activity, higher numbers of lignocellulolytic enzyme gene, and different response of cellulase gene expression to some disaccharides. To boost the economic potential of the biorefineries using lignocellulosic biomass, P. oxalicum strains need to be further improved regarding the performance and production level of the enzyme systems.