Technology has advanced to the point where microseismic monitoring of hydraulic fractures can provide critical information for fracture optimization. Important elements of a monitoring system include the receivers, the telemetry system, and automatic processing of the vast amounts of data. Procedures and additional data requirements are discussed and examples of the important results which can be obtained are illustrated. Hydraulic fracturing is a critical technology for the exploitation of natural gas and oil resources, but its optimization has been impeded by an inability to observe how the fracture propagates and what its overall dimensions are. Recent field experiments in which fractures have been exposed through coring or mineback have demonstrated that hydraulic fractures are not the ideal, symmetric, planar features that are currently envisioned. Instead, they appear to commonly have multiple strands, secondary fractures, height and length asymmetries, and other complexities which make a priori predictions difficult. It is clear that model validation, fluid selection, proppant loadings, problem identification and solution, field development, and many other aspects of fracture optimization have been encumbered by the absence of ground-truth information on fracture behavior in normal field settings. Technology is now becoming available, however, to provide extensive diagnostic information on fracture growth, final size and geometry. Multi-level wireline receiver arrays for downhole passive imaging of fracture behavior have become viable and are demonstrating that hydraulic fractures can be imaged, assessed, and eventually controlled. These receiver arrays require high-quality transducers, well-designed clamping systems, high-speed telemetry, real-time processing capabilities, and careful procedures to be effectively used. This paper discusses this technology, its application and validation, and examples of the value of fracture imaging. It concentrates on a 5-level, accelerometer-based, fiber-optictelemetry system currently being used for microseismic mapping.