The use of hydraulic fracturing has led to substantial increases in the production of oil and gas in the United States. It also has led to numerous new legal issues and technical challenges. One issue that has received significant attention from legal scholars is whether a well owner commits a trespass if hydraulic fracturing fluid travels from the subsurface of a property where the well owner has operational rights into the subsurface of neighboring properties. The Texas Supreme Court concluded that such a subsurface intrusion does not give rise to trespass liability if the only harm alleged by a neighbor is the drainage of hydrocarbons from beneath his property. But a federal court in West Virginia reached the opposite conclusion, ruling that a well owner can be liable for a trespass in such situations. In other states, courts have not yet addressed the issue. Thus, in jurisdictions other than Texas, the potential exists for trespass liability based on subsurface intrusions of fractures. Moreover, even in Texas the potential for liability exists if a plaintiff can prove some harm other than the drainage of hydrocarbons. Accordingly, although much of the attention of engineers and scientists has focused on achieving optimal fracturing designs for known reservoir characteristics, it also is critical that well owners be able to determine the lateral extent of their fractures, both for purposes of disproving any erroneous claims that their fractures have intruded into a neighbor's subsurface, and to provide data to improve their hydraulic fracturing modelling, which may help them design fractures that extend nearly to the border of properties where the well owner has operational rights, without undue risk that fractures will extend beneath neighboring land. In this paper, after briefly examining some of the legal issues that drive the need for fracture delineation, we review current practices used in the industry to determine fracture dimensions using tilt meters, microseismic maps, pressure analysis and simulation, and discuss the inherent uncertainties of these techniques. Finally, we analyze strategies for using technology to minimize the likelihood of legal liability.