Geoarchaeology: An International Journal, Vol. 17, No. 5, 441–451 (2002) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gea.10021  2002 Wiley Periodicals, Inc. ComparisonoftheMaritimeSitesand MonumentsRecordwithSide-ScanSonar andDiverSurveys:ACaseStudyfrom RathlinIsland,Ireland RoryQuinn, 1 WesForsythe, 1 ColinBreen, 1 MartinDean, 2 MarkLawrence, 2 andSteveLiscoe 2 1 Centre for Maritime Archaeology, Coastal Studies Research Group, School of Environmental Studies, University of Ulster, Coleraine, Co. Derry, BT52 1SA, Northern Ireland 2 Archaeological Diving Unit, University of St. Andrews, Fife, KY16 9AJ, Scotland Rathlin Island, off the north coast of Ireland, has a history of settlement and seafaring from the Late Mesolithic period to the present day. The maritime Sites and Monuments Record (SMR) for Rathlin indicates many wrecking incidents. In 1999, a reconnaissance side-scan sonarsurveyconfirmedthepresenceof46targetsofpossiblearchaeologicalpotentialaround Rathlin Island. Thirteen of these anomalies were positively identified as shipwrecks. Of the remaining 33 targets, nine were dived on in order to ground-truth the geophysical data. A successful and rapid methodology of ground-truthing side-scan sonar data for archaeological purposes was developed. The results confirmed the presence of a Danforth Anchor at one site, while the remaining anomalies were identified as geological features. The results from the side-scan survey and diver-truthing exercise enhanced the existing maritime SMR.  2002 Wiley Periodicals, Inc. INTRODUCTION The success of any marine archaeogeophysical survey is largely dependent upon the experience of the surveyors and interpreters. Although some research has fo- cused on the geophysical signatures of anthropogenic materials submerged and buried in the marine environment (Quinn et al., 1997a; Bull et al., 1998; Newell, 1999), it remains a poorly understood subject. This often leads to the misinterpre- tation of acoustic archaeological data. Therefore, the only satisfactory method presently available for quantifying marine archaeogeophysical data is to physically ground-truth any ambiguous reflectors, either through diver or remotely operated vehicle (ROV) investigations. With the increase in predevelopment and research- orientated geophysical surveys (Quinn et al., 1997b, 1998; Arnold et al., 1999; Mom- ber and Geen, 2000; Simms and Albertson, 2000), combined with initiatives to sub- stantiate SMRs, a rapid and accurate method for ground-truthing geophysical data is essential. This paper details the methodology and results of such an exercise