Alignment systems in LHC general purpose detectors

Tracking systems in large multipurpose particle physics detectors are comprised of a large number of active elements designed, for precision measurement of particle trajectories. If detectors are to perform as designed a precise knowledge of the location of the different elements, with precision similar to the intrinsic detector resolution, is required. We will describe the design criteria and design considerations for optical alignment systems that achieve these goals. Issues discussed will include resolution discussions, integrating alignment with tracking detectors, geometric considerations, test beam activities, optical devices, calibration, and software analysis and validation procedures. We will describe the implementation of these ideas mainly in the CMS and ATLAS detectors operating at LHC.