The 1/N expansion

as a rigorous basis of the Mean Field Theory (MFT)

is discussed in several typical polymer systems.It shows that MFT can be applied well in polymer physics only if the system has certain symmetries.In neutral polymer solutions

MFT is suitable for concentrated solutions and melts

in which N is no less than six

but not suitable for dilute and very dilute solutions whose N is less than six.Moreover

MFT is a good description in polyelectrolyte solutions if the polymer chains are very weakly-charged.However

for the case of strongly-charged systems

MFT is no longer the right way due to the low symmetries of the system

then maybe the scaling theory will be a good choice.