In Asia, a variety of dietary products have been used for centuries as popular remedies to prevent or treat different diseases. A large number of herbs and extracts from medicinal mushrooms are used for the treatment of diseases. Mushrooms such as Ganoderma lucidum (Reishi), Lentinus edodes (Shiitake), Grifola frondosa (Maitake), Hericium erinaceum (Yamabushitake), and Inonotus obliquus (Chaga) have been collected and consumed in China, Korea, and Japan for centuries. Until recently, these mushrooms were largely unknown in the West and were considered “fungi” without any nutritional value. However, most mushrooms are rich in vitamins, fiber, and amino acids and low in fat, cholesterol, and calories. These mushrooms contain a large variety of biologically active polysaccharides with immunostimulatory properties, which contribute to their anticancer effects. Furthermore, other bioactive substances, including triterpenes, proteins, lipids, cerebrosides, and phenols, have been identified and characterized in medicinal mushrooms. This review summarizes the biological effects of Ganoderma lucidum upon specific signaling molecules and pathways, which are responsible for its therapeutic effects.