In recent years, a growing number of consultants and owners have considered the use of horizontal remediation wells as an alternative solution to the removal and containment of contaminants. Currently, engineers rely on complex systems of vertical wells or barrier wall systems which inject and/or extract subsurface water or vapor for on-site soil and ground water remediation. However, in sites containing fine-grained soil or wide spread contamination, numerous vertical wells are required to adequately treat the plume. Horizontal directional drilling makes it possible to install a single horizontal well to contact a much larger contaminated area. Because fewer wells are required, the cost of extraction and treatment over the life of the project may be substantially reduced, In addition, horizontal wells can be installed beneath buildings or environmentally sensitive areas, to reach contaminants inaccessible by other methods. Construction of such wells may be achieved using horizontal directional drilling, a remotely guided drilling technique that makes it possible to drill horizontal boreholes over long distances along straight or curved profiles and alignments. An overview of horizontal environmental well technology including discussion of the advantages and limitations of this technology is presented. Greater utilization of horizontal wells depends on the availability of design and construction guidelines. Parameters and design principles that should be considered in such installations are presented.