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Underground longwall mining results in both vertical and horizontal ground displacements. Although major studies have been conducted to evaluate vertical displacement component (subsidence), the overall examination of the horizontal displacement of the ground surface has been limited. In this study, investigations were conducted to evaluate the behavior of horizontal ground displacement due to longwall mining. As the magnitude and direction of horizontal ground displacement depend on the surface topography, both flat and rugged terrains were considered in the study. A correlation was developed between horizontal ground displacement and other site and displacement parameters by studying subsidence data of a mined area in the State of Wyoming as well as data collected from other underground mines in other mining regions. Based on the results, in flat grounds, the induced horizontal ground displacements are essentially perpendicular to the subsidence contours with the maximum value typically between 0.20 and 0.35 of the maximum subsidence. An empirical correlation was suggested to determine the horizontal ground displacement using ground subsidence data. For rugged terrains, correlations of the measured horizontal displacements were difficult to find but appear to be related to induced slope instability. Based on these findings, these displacements are directed down slope and have a greater magnitude than the displacements determined in flat terrains. [ABSTRACT FROM AUTHOR]