Abstract: With the rapid development of biologics, there is an increasing tendency for drugs to be developed in a patient-friendly manner. Subcutaneous administration has drawn the attention of researchers because of its ability to be used for self-injection. High viscosity and poor stability are two main challenges in the process of developing highly concentrated antibodies that can be used for subcutaneous administration, yet current studies have focused on both challenges separately, without approaching their mutual influence to explore the solution that can address the problem of high viscosity while maintaining stability. To deepen knowledge and comprehension of the main challenges in highly concentrated antibodies, and to find a solution to the problems of high viscosity and poor stability, this review outlines the principles and internal relationship between high viscosity and poor stability from the standpoint of protein-protein interactions. Besides, potential solutions that can reduce viscosity while maintaining stability in highly concentrated antibodies are discussed on the basis of recent research advances and marketed highly concentrated formulations.