Since the start of the 21st century, we have seen enormous progress in the development of a new generation of optical atomic clocks. Based on narrow optical transitions in trapped laser cooled atoms or ions, these have demonstrated unprecedented frequency stability and estimated systematic frequency uncertainty, far surpassing the current generation of caesium microwave primary frequency standards. The accuracy with which frequency measurements can be made is thus limited by our present definition of the second in the International System of Units, rather than by the performance levels reached by the technology.
The international community is therefore anticipating change, and a special taskforce set up by the Consultative Committee for Time and Frequency has prepared a roadmap towards a redefinition of the second in terms of an optical transition or transitions. This roadmap sets out a set of mandatory criteria that must be met before redefinition, as well as some auxiliary conditions related to work that should be well advanced at that time.

In this presentation I will review the progress that has been made over the past decade towards fulfilment of these criteria, including the verification of optical clock uncertainties via international comparisons and the incorporation of optical clocks into national and international time scales. I will present the different options that are being considered for the redefinition and conclude with a perspective on the challenges that still lie ahead.