A range of techniques are widely used to investigate the crystallisation process: Traditional crystallographic methods to characterise crystalline structures, such as small-angle and wide-angle X-ray scattering, X-ray spectroscopy, and X-ray diffraction; spectroscopic techniques including nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy provide insight into the chemical structure and the shape of the molecules during crystallisation; ultraviolet-visible (UV-Vis) spectroscopy can measure the solute concentration but are of limited use when nascent particles result in scattering losses that cannot be distinguished from absorption. Turbidimetry is used to measure the loss of intensity of transmitted light due to the scattering effect of those particles. Second-harmonic generation and polarised light microscopy are applied to detect the onset of crystallisation but are not very sensitive to the structure. When investigating crystallisation in aqueous solutions, the strong absorption due to the presence of water makes it impossible to perform FTIR measurements in transmission. Instead, observations are restricted to surface measurements using attenuated total reflection (ATR). While Raman spectroscopy does not suffer from this restriction, the lack of interaction means that only very little information, if any, of the solvent molecules during the crystallisation process can be extracted. Terahertz time-domain spectroscopy (THz-TDS) offers a unique perspective to characterise the crystallisation process both in terms of information from the solvent as well as the emerging crystals. By measuring the amplitude and phase of single-cycle pulses of farinfrared radiation, in solution, THz-TDS can probe large-amplitude inter-molecular vibrations as well as high-frequency dielectric relaxation processes that correspond to relaxation times of less than 10 ps. In solids, the technique can distinguish between different polymorphic forms, cocrystals, hydrates and solvates, as well as provide an excellent measure for overall crystallinity and defect density crystallinity since the long-range order in crystals results in well-defined spectral features (fingerprints) in the terahertz region. In contrast, in amorphous materials, the lack of long-range order results in the collapse of the well-defined peaks into a vibrational density of states (VDOS), which starts at a few hundred gigahertz and exceeds the entire spectral bandwidth of modern THz-TDS spectrometers (0.3 THz to 3 THz). It is characterised by a featureless, monotonously increasing absorption coefficient that typically peaks at frequencies beyond 3 THz. The deconvolution of individual atomic contributions in experimental terahertz spectra is not possible without additional information, which is usually gathered from theoretical simulations. Density functional theory (DFT) simulations provide normal mode vectors and force constants and can therefore be used to investigate and visualise vibrational modes. Studies of water and water/alcohol mixtures with THz-TDS suggested key concentration transition points that marked different stages of water and alcohol molecular interactions. Other water mixtures and solutions also demonstrated the use of THz-TDS to probe water molecules based on their mobilities and the behaviour of the hydration shell. As well as for characterising static structures, THz-TDS has also been found useful to probe reaction dynamics, such as solid-solid phase transitions, amorphous-solid transformations, and crystallisation. Previously, terahertz spectroscopy has been applied to study the crystallisation of sugar and L-(+)-tartaric acid utilising attenuated total reflectance geometry and triggering the crystallisation process by the evaporation of water from the aqueous solution. During crystallisation the VDOS is depleted, resulting in the emergence of peaks and a dropping of the overall baseline given its nature as the flank of the VDOS. THz-TDS can therefore simultaneously probe amorphous and crystalline phases represented by the behaviour of the baseline and peaks, respectively. In addition, this suggests that the behaviour of liquid phase can be extracted explicitly from the baseline while crystallising, hence the solute concentration can be measured even in semi-crystalline samples. MgSO 4 is chosen as a model system to demonstrate that THz-TDS is an option to complement the currently widely applied tools in the field of crystallisation. The versatile set-up based on THz-TDS in transmission geometry and the methodology described in detail in Li et al. and Kölbel et al. is further used to observe the dissolution of crystals at elevated temperatures, calculate the equivalent local concentration, and can be extended to other systems of interest.