The pre-Christian royal site of Rathcroghan (ancient Cruachain) is situated about 5 km to the northwest of the village of Tulsk in Co. Roscommon, Ireland. The archaeological remains of over 50 visible monuments, of various type and antiquity, are concentrated over an area of some 10 km(2). prominently located at its approximate centre is Rathcroghan Mound, averaging 88 m in diameter and 6 m in height. The Archaeo Geophysical Imaging Project, funded by the Heritage Council, has completed a major programme of archaeological and geophysical surveys at Rathcroghan, concentrating primarily on Rathcroghan Mound.Detailed microtopographic surveys have enabled the production of computer generated maps and terrain models. These have aided the identification of a number of low-relief features that had remained unnoticed during reconnaissance field inspection. Magnetic susceptibility survey has delimited zones of activityon the top of the mound and has also identified an ancient field system surrounding it. Magnetic gradiometry survey, conducted at differing sample resolutions, complements and extends the results of the magnetic susceptibility survey. An enclosure, 370 m in diameter, is identified encircling Rathcroghan Mound and a number of its satellite monuments. Also identified are the subsurface remains of foundation trenches for a series of large-scale, circular structures or enclosures, approximately 20 m to 32 m in diameter, which once occupied the summit of the mound. Another, 26 m in diameter, lies immediately to its north-northeast. Electrical resistance survey on Rathcroghan Mound, using different array types, confirm the presence of a series of circular and arcuate features, many of which correspond to those noted in the magnetic surveys.The compelling results of the Rathcroghan survey have enabled more extensive and meaningful archaeological comparisons to be made between this and the other major, pre-Christian, royal centres of Tara (Temhair), Co. Meath, Navan Fort (Emain Macha), Co. Armagh and Knockaulin (Don Ailinne), Co. Kildare. Copyright (c) 2005 John Wiley & Sons, Ltd.