Aside from drying techniques, understanding the specific characteristics that influence the delivery and stability of protein drugs is also crucial in the development of biological products. This knowledge serves as a fundamental basis for comprehending the underlying mechanisms involved in the development process. Analytical tools, specifically those in structural biophysics and dynamics, including Fouriertransform infrared spectroscopy (FTIR), neutron scattering (NS), dielectric spectroscopy (DES), Nuclear Magnetic Resonance (NMR), have been increasingly utilized to characterize protein structure and motion in the solid states. For example, NMR spectroscopy, both solution and solid-state NMR, is a sophisticated technique used in the biophysical analysis of proteins. It has been utilized to study various aspects of protein behavior such as conformational stability, interactions between proteins, complexes formed between proteins and excipients, as well as the arrangement of molecules in insoluble fibrillar aggregates. These studies aid the development of rational formulation designs from molecular mechanism understanding. Theories regarding the mechanism by which sugars or polyols stabilize protein both during drying and storage have been proposed and widely discussed in the literature, including vitrification theory, the water replacement theory, and refinements focusing on the local mobility of protein. However, the mechanism of dry-state biologics stabilization is still not completely understood. Hence, we will provide an overview and clarification of up-to-date protein stabilization mechanisms or hypotheses in the solid state. Such mechanistic insights via advanced characterization can enable better insights into the impact of formulation design on protein structure and stability, which ensures the ultimate quality of the protein solid products.