Raman spectroscopy is often used to determine the composition and strain in Si_1-xGe_x films and structures. It is a fast, non-destructive technique that can give results with a spatial resolution at the micron-scale, and even information on nm-sized structures. In principle, the intensity and frequency of the three Raman peaks associated with vibrations of Ge-Ge, Si-Ge and Si-Si bonds offer information on strain and composition. However, this is only straightforward for compositions where all three, or minimal two of these three Raman peaks can be detected, typically between x = 0.2 and 0.8. And even for that case, although there is consensus about the equations to be used, various publications give different values for the constants to be used in these equations. In addition, the Si-Ge Raman peak makes the evaluation difficult, because its position and shape depends on the number and position of the nearest neighbors of a Ge or Si atom in a Si or Ge host lattice, respectively. There is not one, but there are several different kinds of Si-Ge Raman peaks. If less than 1% Si atoms are present in Ge, there is a localized Si phonon mode related to vibrations of an isolated Si atom surrounded entirely by Ge atoms, and a different method has to be used in Raman spectroscopy to obtain information on composition and strain (see figure). This talk discusses these challenges and proposes potential solutions.