Abstract: Quantifying coherence is an essential endeavour for both quantum foundations and quantum technologies. I will give an overview of recent developments in the characterisation and quantification of coherence as a resource. In particular, I introduce the robustness of coherence as a mathematically rigorous and physically motivated quantifier of quantum coherence, which complies with the requirements dictated by the resource theory of coherence. The measure is shown to be observable, as it can be recast as the expectation value of a coherence witness operator for any quantum state. Finally I discuss an operational interpretation: the robustness of coherence quantifies the advantage enabled by a quantum state in a phase-discrimination task.