The research in the field of aqueous two phase extraction for proteins for further downstream processing as biopharmaceutical molecules, has gained the attention of many groups. Most of these systems are based on polyethylene glycol (PEG) and other salts, especially phosphates. However, denaturation of proteins, long processing stages as well as the requirement for further purification stages due to certain used salts such as phosphates is still a challenge, which requires more improvement. The designing of alternatives for such phosphate-based systems is the focus of this book, and is considered an extension of new efforts for improving the conditions of downstream processing of proteins.