One class of the recently developed subspace identification algorithms identifies stochastic linear time-invariant systems. Therefore these algorithms are useful for the determination of the modal parameters of a vibrating structure excited by unmeasured ambient loading. After the introduction, the stochastic subspace identification technique is explained. The relation between on the one hand, the model of the vibrating structure and on the other hand, the stochastic system model is made clear. As a side result of this relation the modal parameters are extracted from the system matrices. Finally the technique is applied to experimental data generated under both laboratory and in-situ conditions.