  The rapid increase in accuracy and stability of optical atomic clocks
compared to the caesium atomic clock as primary standard of time and frequency
asks for a future re-definition of the second in the International System of
Units (SI). The status of the optical clocks based on either single ions in
radio-frequency traps or on neutral atoms stored in an optical lattice is
described with special emphasis of the current work at the
Physikalisch-Technische Bundesanstalt (PTB). Besides the development and
operation of different optical clocks with estimated fractional uncertainties
in the 10^-18 range, the supporting work on ultra-stable lasers as core
elements and the means to compare remote optical clocks via transportable
standards, optical fibers, or transportable clocks is reported. Finally, the
conditions, methods and next steps are discussed that are the prerequisites for
a future re-definition of the second.
