This chapter describes the theoretical methods and experimental methods for investigating the properties of helium in metals. The methods listed are field ion microscopy (FIM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal desorption (THDS), electron energy loss spectroscopy (EELS), perturbed γγ angular correlation (PAC), neutron and X-ray small-angle scattering (SAS), positron annihilation (PA), nuclear scattering methods (NSC), and differential dilatometry (DD). Most of the techniques are common to many fields of solid state research. An exception is thermal helium desorption spectroscopy (THDS) whose principle is thus briefly described. The method is based on the measurement of the rate of helium released from a specimen whose temperature is increased by a given rate. From the corrected thermal desorption spectra, the following quantities are usually extracted: temperatures of maximum release (peak temperatures), half-widths of the peaks and maximum release rates. These quantities are in turn related to the microscopic parameters governing the physical processes leading to the observed release peaks such as type of reaction. Although the evaluation of THDS-data by this procedure is not always free of ambiguities, it is up to now the only method -besides field ion microscopy-which yields information about the atomistic properties of helium in solids.