Behaviour of light non-aqueous phase liquids (LNAPLs) within a fractured rock mass is a function of the properties of the immiscible fluid, the fracture network, rock matrix properties, and the groundwater regime. LNAPL behaves differently in rock with open fractures than it does in porous media. Relatively small volumes of LNAPL within vertical or subvertical fractures can produce significant LNAPLpressure heads, resulting in LNAPL penetration into the saturated zone. Penetration can be significantly deeper than predicted by porous medium models. Groundwater surface fluctuations can cause lateral LNAPL migration, even up-gradient to natural gradients. Characterization of LNAPL-contaminated fractured rock masses must take into account these fundamental differences in behaviour. Site investigation should focus on determination of fracture network and rock matrix properties, understanding of groundwater surface fluctuation dynamics, and consideration of spatial LNAPL distribution. A combination of techniques, many not used in porous medium investigations, but can used to develop a detailed conceptual model. These include coring, angled holes, digital borehole imaging, and fracture casting for aperturedetermination. The data provide information on LNAPL occurrence and behaviour, allow LNAPL spill volume to be estimated, indicate future movement, and ultimately allow for more effective and economic remedial decision making.