The tetrahydroborates represent a class of complex hydrides with the largest gravimetric and volumetric hydrogen density for hydrogen. Therefore, these compounds are potential hydrogen storage materials for mobile applications. The alkali metal and alkali earth metal tetrahydroborates have been known to exist for more than 50 years, and their chemical properties have been investigated intensively. However, their physical properties are to a large degree still unknown. The main physical parameters, e.g., structure, stability and hydrogen diffusion, are summarized and discussed in this paper.