Cracking is one of the most common issues in practically all reinforced concrete (RC) structures. The complexity of this mechanism is influenced by multiple actions and phenomena – from early-age concrete strains during hydration, through the effects of temperature, shrinkage, creep, and finally mechanical loading. It can also vary depending on the type of concrete used, the static scheme, or operating conditions. Theoretical analysis is challenging, and even with an interdisciplinary approach that integrates materials science and structural engineering, the results are subjected to high uncertainties. Therefore, reliable monitoring tools for crack detection, localisation, and width estimation are crucial for accurate structural assessment. Distributed fibre optic sensing (DFOS) is a unique technology that enables the measurement of selected physical quantities continuously along its entire length, making direct damage (and crack) detection possible. Integrating sensors with the structure before concreting opens up new possibilities in structural diagnostics. This article summarises recent achievements in the field and presents a set of practical case studies.