Following the appearance of ionic liquids (ILs) and deep eutectic solvents (DESs), natural deep eutectic solvents (NADESs) have emerged as a new type of truly green solvents with many excellent advantages such as cheapness, sustainability, biocompatibility, environmental friendliness, and, in particular, remarkable solubilizing power and outstanding designability. Although only at an early stage, research on NADESs has started to blossom with exponential growth, showing attractive and promising potentials for applications in various areas. In this chapter we focus on an introduction to what is currently known about NADESs: their formation, structure and roles in nature, their physical/chemical properties, their toxicity and biodegradability, and, more importantly, their beneficial applications in biotechnology.