A new and efficient method is introduced for computing transition pathways, free energy barriers, and transition rates in complex systems with relatively smooth energy landscapes. The method involves evolving strings, which are smooth curves with intrinsic parametrization, to find the most probable transition path between metastable states. Free energy barriers and transition rates are then determined using umbrella sampling around the string. The method is applied to Lennard-Jones cluster rearrangement and thermally induced switching of a magnetic film. The dynamics of complex systems are often driven by rare events, such as nucleation during phase transitions, conformational changes in macromolecules, and chemical reactions. These events are slow due to the large energy barriers relative to thermal energy. The method uses strings to find the most probable transition paths, which are then used to compute free energy barriers and transition rates. The string method is compared to the nudged elastic band (NEB) method. While NEB uses a spring force to ensure equal spacing, the string method uses an intrinsic parametrization, allowing for more flexible and efficient computation. The string method can be generalized to infinite-dimensional systems by introducing an appropriate norm in phase space. The method is applied to a seven-atom Lennard-Jones cluster, where the string method successfully finds the most probable transition path. The method is also applied to magnetic films, where it identifies two generic switching mechanisms: domain wall motion and vortex nucleation. The string method is efficient and allows for sampling of configuration space in regions otherwise inaccessible by standard Monte Carlo methods. It uses an intrinsic parametrization, leading to a simple and efficient algorithm for solving the evolution equation. The method is validated by comparing results with known values and shows faster convergence than traditional methods. The transition rates are computed using thermodynamic integration and Kramers' theory, demonstrating the method's effectiveness in determining rare event dynamics.A new and efficient method is introduced for computing transition pathways, free energy barriers, and transition rates in complex systems with relatively smooth energy landscapes. The method involves evolving strings, which are smooth curves with intrinsic parametrization, to find the most probable transition path between metastable states. Free energy barriers and transition rates are then determined using umbrella sampling around the string. The method is applied to Lennard-Jones cluster rearrangement and thermally induced switching of a magnetic film. The dynamics of complex systems are often driven by rare events, such as nucleation during phase transitions, conformational changes in macromolecules, and chemical reactions. These events are slow due to the large energy barriers relative to thermal energy. The method uses strings to find the most probable transition paths, which are then used to compute free energy barriers and transition rates. The string method is compared to the nudged elastic band (NEB) method. While NEB uses a spring force to ensure equal spacing, the string method uses an intrinsic parametrization, allowing for more flexible and efficient computation. The string method can be generalized to infinite-dimensional systems by introducing an appropriate norm in phase space. The method is applied to a seven-atom Lennard-Jones cluster, where the string method successfully finds the most probable transition path. The method is also applied to magnetic films, where it identifies two generic switching mechanisms: domain wall motion and vortex nucleation. The string method is efficient and allows for sampling of configuration space in regions otherwise inaccessible by standard Monte Carlo methods. It uses an intrinsic parametrization, leading to a simple and efficient algorithm for solving the evolution equation. The method is validated by comparing results with known values and shows faster convergence than traditional methods. The transition rates are computed using thermodynamic integration and Kramers' theory, demonstrating the method's effectiveness in determining rare event dynamics.