The condition of a concrete structure can be effectively assessed through the monitoring of cracks. Large cracks may be warning signs of severe degradation, while small cracks with openings from 0.2 to 0.4 mm may lead to durability problems associated with the penetration of water and other chemicals. In this presentation, we will describe recent developments on a fiber optic crack sensor that allows the detection and monitoring of multiple cracks without requiring prior knowledge of crack locations. After a discussion of the sensing principle, a theoretical model for signal loss vs crack opening is derived to provide guidelines for sensor design. The fabrication of a sensor suitable for both external bonding and internal installation in a concrete structure will then be described. Representative experimental results will be shown and comparison with theoretical predictions is conducted. Based on the experimental and theoretical findings, the potential of the sensor for practical applications is demonstrated.