Understanding the mechanism of how a solid matrix can stabilise a dry-state protein is essential for developing predictive parameters against aggregation and degradation processes . In the literature, several studies into the dynamics of solid-state proteins examined the coupling of protein internal dynamics to the dynamic properties of a host matrix, and a selection of excipients which slowed down protein dynamics at low temperatures were identified . A number of dielectric relaxation processes have been described in the literature at least two of which are observed by all amorphous molecular materials, including proteins: the primary, or ↵-relaxation process, associated with large-scale mobility and the secondary, or -relaxation, associated with local mobility, or small-scale mobility . Notably, the Johari-Goldstein (JG) -relaxation process is considered a universal feature of all amorphous materials and is widely regarded to be dominated by the intermolecular degrees of freedom of a molecular material . The potential energy surface (PES) model proposed by Goldstein half a century ago can be used to understand the molecular dynamics in amorphous systems, and that intra-and intermolecular processes are always fundamentally coupled by means of the PES . Given that neutron scattering is sensitive to processes on the order of a nanoseconds or faster, one study utilised neutron scattering experiments in order to establish a link between the rate of protein degradation and the the -relaxation dynamics measured by the mean squared displacement, hu 2 i, for a a series of lyophilised formulations . Specifically, the amplitude of the fast -process,which has been associated with motions occurring on the timescale of picoseconds, is proportional to hu 2 i ( . Numerous studies have been completed with the motivation of understanding the -relaxation dynamics in order to utilise it as a parameter and a robust metric for predicting protein stability; however, the majority of these works have been completed using neutron back scattering, and these experiments cannot be implemented for routine formulation analysis due to time and cost limitations . Dielectric spectroscopy which probes processes in the 10 6 and 10 12 seconds range and MHz/GHz frequency range, has been used to understand the dynamic relaxation processes of protein-solvent interfaces . Techniques such as circular dichroism (CD) spectroscopy and X-ray crystallography are typically used to study the structure of proteins in solution and in crystalline solids. CD spectroscopy requires samples to be dissolved and crystallography is of limited use in studying the amorphous powders produced by lyophilisation . Moreover, vibrational spectroscopy methods such as Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy can be used to probe the secondary structure of a protein after lyophilisation, and how they are affected by the presence or absence of surfactants and stabilising excipients. Nonetheless, dielectric spectroscopy and neutron scattering experiments are considered the optimal techniques for investigating protein stability given that these techniques are sensitive to larger time scales, and thus it is essential to investigate protein motions above and below the glass transition temperature (T g ) as these are thought to play a critical role in protein stability. Similar to neutron scattering, THz-TDS can be used to measure molecular mobility at the relevant length and timescales over a broad temperature range by probing molecular dipoles at frequencies of 0.1 3 THz. Using THz-TDS, at the glass transition temperature (T g ) we can observe the T g,↵ associated with the ↵-relaxation process, and below T g , we can observe T g, associated with the -relaxation process . In doing so, THz-TDS can be used to characterise the molecular mobility of a system, which is a key factor when determining the chemical stability of a formulation, as an increase in molecular mobility has been directly linked to an increase in chemical degradation of a material and its storage stability. Thus, using THz-TDS it is possible to investigate the interplay between dielectric and vibrational dynamics and study the molecular mobility and structural dynamics of lyophilised formulations.