Permanent electric dipole moments (EDMs) of physical systems violate 
both parity (P) and time-reversal (T) symmetries. By CPT invariance, 
they equivalently point to the much sought-after charge-parity (CP) 
violation, which is an important feature of understanding the baryon 
asymmetry of the universe and numerous extensions of the Standard Model. 
Active EDM searches cover a wide variety of systems including neutron, 
nuclei, atoms, molecules, and solids, with most of them being charge 
neutral. In this talk, I will revisit the famous Schiff theorem, which 
accounts for the re-adjustment of charge constituents in a neutral 
system to screen out the detecting electric field hence reduces the EDM 
response, and then discuss the issues pertaining to the interpretation 
of atomic EDM measurements.