The design of a Cooperative Adaptive Cruise Control (CACC) system and its practical validation are presented. The proposed CACC system includes a wireless communication link with the nearest preceding vehicle. The CACC design targets string-stable system behavior for a platoon of vehicles driving at small inter-vehicle distances. The string-stability characteristics of the system are assessed using a frequency-domain approach. Considering a velocity-dependent inter-vehicle spacing policy, it is shown that the communicated information enables small inter-vehicle distances, while maintaining string-stable behavior, whereas for a constant, velocity-independent inter-vehicle spacing, string stability can not be guaranteed. To validate the theoretical results, experiments are performed with two CACC-equipped vehicles.