The chapter focuses on the formulation of novel solvents, namely, deep eutectic solvents (DESs), especially in the energy and environmental domains. DES are found to be superior in their number of applications compared to conventional organic solvents or ionic liquid due to their lower vapor pressure, high thermal stability, and lower associated economic costs. The ability to tune DESs by adjusting the hydrogen bond donor (HBD) and the hydrogen bond acceptor (HBA) has granted the flexibility to use them in various applications. They have promise as a new family of mixtures with much lower melting temperatures than their constituent components. These materials show promise as novel “designer” solvents with variable physicochemical features. The hydrogen bonding between the HBD and the HBA is hypothesized to be the root cause of their lower melting point and their physicochemical features. This chapter focuses on the current academic and industrial attempts to help build a basic framework for understanding DESs. It summarizes current DES research, highlights remaining scientific concerns, and suggests research directions to expand the field’s predictive models along with it the basic knowledge of these solvents.