Single-crystal films were simulated using three-dimensional discrete dislocation dynamics simulations, where an initial distribution of dislocation loops was allowed to move naturally in response to successive applied strains. The types of interactions that stopped threading dislocations (threads) were identified and the relative fraction of threads stopped in each interaction was determined. An inhomogeneous stress field in the film evolved as the dislocation structure evolved. Threads were observed to interact primarily in regions of low stress. The simulations were used as a virtual test bed for understanding dislocation behavior in thin films. The intuition gained from the simulations led to the construction of three models, which are discussed in detail. First, a model was developed to determine the capture cross-section of a thread, such that if another thread was within its capture cross-section the two threads would interact. Second, a statistical model was constructed to evaluate the effect of stress inhomogeneity on the local concentration of threads. Finally, results from the simulations and analytical models were used to construct a model of strain hardening in thin films based on fundamental behavior of dislocations in thin films.