Broadband spectroscopy combines wide spectral coverage with high resolution and accuracy, requirements that are often difficult to satisfy simultaneously using conventional approaches. Dual-comb spectroscopy addresses this challenge by using two optical frequency combs with slightly different repetition frequencies to map optical spectra directly into the radio-frequency domain. The method relies on time-domain interferometry and avoids mechanical scanning, enabling precise, rapid and broadband measurements. In principle, the achievable spectral resolution is set by the comb line spacing, defined by the laser repetition frequency. Because the measurement does not rely on geometrical constraints, the dual-comb interferometer offers a conceptual route towards broadband spectroscopy with resolution determined purely by temporal coherence. Over the past two decades, dual-comb spectroscopy has been implemented across the electromagnetic spectrum, from the terahertz to the visible, with ongoing efforts towards the ultraviolet. In this Primer, we present the fundamental principles of the technique, analyse the parameters governing its performance - including resolution, accuracy, signal-to-noise ratio and acquisition speed - and describe practical approaches to data acquisition and processing. We conclude by examining representative applications, current limitations and emerging directions for further development.
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