:Dear Nanite, it is a good point (which I did not realize) that spatial dispersion describes one specific sort of nonlocal behaviour that exists in homogeneous infinite media only. In fact, spatial dispersion is relatively common in optics, one example being Doppler broadening in gases, but it can often be approximated by frequency dispersion.  
 :As a result, I believe SD gets less attention than it would deserve in E&M; courses. When one holds a hammer, all problems look like a nail; when a generation of physicists is taught the local theory, almost everybody tries to describe metamaterials in terms of local parameters. This however causes much confusion, because as far as I know, above GHz range, we are only able to make metamaterials that are spatially dispersive (and lossy). If no fundamental breakthrough comes, I guess it will remain true in the next decades. 
 :I therefore happily support the idea of writing a robust and readable article on spatial dispersion, though I can not promise to invest much time in it now. The article can in general relate to any waves propagating in a (nonlocal homogeneous) medium, so the theory can be composed of wave equation and convolution with nonlocal response function. The second part of the article could focus on the electromagnetic wave, extending the subject to the electromagnetic "L&L; §§103-106" theory () and mentioning several examples, including gases and metamaterials. What do you think about this?