At the 2014 General Conference of Weights and Measures a revision was
proposed to dramatically improve the robustness of the definitions of
the kilogram, Ampere, Kelvin, and mole, by linking them to exact
numerical values of the Planck constant h, elementary charge e,
Boltzmann constant k, and Avogadro constant NA. The hope is that this
change will be enacted at the next conference (2018).
The following superb little video explains clearly why this needs to be
done, and illustrates (very clearly, I think) how it is most likely
going to be achieved in the case of the kilogram.