The advanced surveillance testbed was developed to provide high-fidelity simulation of space- based electro-optical sensors, their associated signal processing, and multi-target tracking algorithms as well as the testing/evaluation of the algorithms. It has a modular design for ease of expansion and maintenance. The testbed begins with scenario definition which includes the definition of satellite constellations, specification of targets, the modeling of atmospheres, and the selection or generation of background scenes. Existing sensor modules include an advanced staring sensor, two linear scanners of advanced design, and a simplified generic sensor for quick simulations having lower fidelity. Modules for two-dimensional (monocular) tracking include a highly specialized tracker for use with the staring sensor and several two- dimensional trackers that can use angle-intensity data from any source. Results of two or more monocular simulations can be combined as inputs to algorithms for data fusion and three- dimensional tracking. `Optimal' two- and three-dimensional tracking algorithms are also provided. Additional modules provide overall control, graphics, and track analysis. This presentation discusses the testbed and its capabilities describing the models with emphasis placed on the background modeling and the implications of background and the lessons learned.