A hagioscope (from Gr. άγιος, holy, and σκοπεῖν, to see) or squint is an architectural term denoting a small splayed opening or tunnel at seated eye-level, through an internal masonry dividing wall of a church in an oblique direction (south-east or north-east), to enable one or more worshippers in side-chapels, private manorial chapels, chantry chapels at the east ends of the aisles, or other parts of the church from which the high altar in the chancel was not visible, to view the elevation of the host, in Roman Catholic and pre-Reformation usage, the most sacred part of the mass at which point a bell was rung and the congregation was required to make the sign of the cross. Where such areas were separated from the high altar not by a solid wall of masonry but by a transparent parclose screen, a hagioscope was not required as a good view of the high altar was available to all within the sectioned-off area concerned. Where a squint was made in an external wall so that lepers and other non-desirables could see the service without coming into contact with the rest of the populace, they are termed "leper windows".
Where the congregation of a church is united in the nave there is no use for a hagioscope. However when parts of the congregation separated themselves for purposes of social distinction, by use of walls or other screens from the chancel, or nave, and from the main congregation, such a need arose. In medieval architecture hagioscopes were often a low window in the chancel wall and were frequently protected by either a wooden shutter or iron bars. Hagioscopes are found on one or both sides of the chancel arch; in some cases a series of openings has been cut in the walls in an oblique line to enable a person standing in the porch (as in Bridgwater church, Somerset) to see the altar; in this case and in other instances such openings were sometimes provided for an attendant, who had to ring the Sanctus bell when the Host was elevated.
Though rarely encountered in continental Europe, they are occasionally found to serve such purposes as allowing a monk in one of the vestries to follow the service and to communicate with the bell-ringers. Sometimes squints were placed to enable nuns to observe the services without having to give up their isolation. The unusual design of the church of St Helen's in Bishopsgate, one of the largest surviving ancient churches of London, arose from its once having been two separate places of worship: a 13th-century parish church and the chapel of a Benedictine convent. On the convent side of the church[which?] a "squint" allowed the nuns to observe the parish masses; church records show that the squint in this case was not enough to restrain the nuns, who were eventually admonished to "abstain from kissing secular persons", a practice to which it seems they had become "too prone".
In France the hagioscope of Notre Dame in Dives-sur-Mer, Normandy, has the inscription trou aux lépreux (leper window). Other hagioscopes are known at St. Laurent in Deauville, Normandy and at the old church of St. Maurice in Freyming-Merlebach, Lorraine.
In Germany a number of hagioscopes still exist or were rediscovered in the 19th and 20th century. They are found mainly in Lower Saxony which had a small population in the Middle Ages and only a few bigger cities. In cities lepers lived together in housing estates which often had their own chapels. In Georgsmarienhütte the hagioscope of church St. Johann belonged to the former Benedictine convent Kloster Oesede, founded in the 12th century and reconstructed in the early 1980s. Nearby in Bad Iburg a hagioscope was rediscovered at St. Clemens, church of former Benedictine monastery in the castle and monastery complex Schloss Iburg. Other hagioscopes in Lower Saxony are found in Bokelesch, Westoverledingen, Dornum, Midlum, Kirchwahlingen (Gemeinde Böhme) and Hankensbüttel.
In Northrhine-Westphalia St. Antonius-Kapelle in Gescher-Tungerloh-Capellen has a hagioscope. St. Antonius is used as Autobahn chapel at Bundesautobahn 31. Another hagioscope is found in St. Ulricus in Börninghausen. In Rhineland-Palatinate the church of St. Eligius-Hospital in Neuerburg has a hagioscope. In Baden-Württemberg there is a hagioscope in St. Peter und Paul, the Old Cemetery Church of Nusplingen.
- A leper's squint (now blocked up) is visible at St Mary's Cathedral, Limerick.
- Athenry Priory also once had a leper's squint
- St Mary's Church, Inis Cealtra has a stone opening believed to be a leper's squint
In Sweden Bro church near Visby on Gotland has a cross-shaped hagioscope. Other hagioscopes are at the church of Vreta Abbey near Linköping, Granhult Kyrka in Uppvidinge and Husaby Kyrka in Götene. The wooden church in Granhult (Småland) has a hagioscope which can be closed.
Churches in England with hagioscopes include:
- St James' in Great Ormside, Cumbria
- St Bees Priory, St Bees, Cumbria (now infilled)
- St Marys Bridge Chapel, Derby
- St Mary's, Lytchett Matravers, Dorset – a particularly large example
- St Martin's Church in Wareham, Dorset
- St Mary and St Cuthbert, Chester-le-Street, Durham
- St Mary’s Church, Easington, County Durham
- St Nicholas' Church, Berden, Essex
- St. Laurence and All Saints Church, Eastwood, Essex
- St Andrew and St Bartholomew's Church, Ashleworth, Gloucestershire.
- St Nicholas Church, Westgate Street, Gloucester, Gloucestershire
- Church of the Holy Rood, Holybourne, Hampshire
- St. Cuthbert's Church, Aldingham, Lancashire
- St James' The Less, Sulgrave, Northamptonshire.
- St Mary's Church, Grendon, Northamptonshire – see above picture
- St Cuthbert’s Church, Beltingham, Northumberland
- St Aidan, Bamburgh, Northumberland
- St Oswald's Church, Sowerby, North Yorkshire
- St Peter's in Upton, Nottinghamshire
- St Nicholas' Church, Old Marston, Oxfordshire
- The Church of St. James, Shere, Surrey
- St Nicholas' Church, Kenilworth, Warwickshire
- St Nicholas Church, Walcot, Lincolnshire
- Holy Trinity Church in Bradford on Avon, Wiltshire
- St Egwin's, Norton and Lenchwick, Worcestershire (now infilled)
- St Mary's Church, Whitby
- St Cuthbert's, Halsall
- St Thomas à Becket Church, Lewes 
At St Bees Priory a purpose-built squint was included in the wall of the 14th-century chapel to give a view of the high altar. The window was low enough to allow a person to kneel whilst looking through the aperture. The hagioscope at St Mary the Virgin, Lytchett Matravers, is unusually large; although unknown in origin it provides a view to the communion table from the 16th-century north aisle. It is large enough that it is often used as a corridor for access to the chancel.
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Hagioscope". Encyclopædia Britannica. 12 (11th ed.). Cambridge University Press. p. 817.
- Kettler, Sarah; Trimble, Carole (2001). The Amateur Historian's Guide to Medieval and Tudor London, 1066-1600. Capital Books. p. 113. ISBN 1892123320.
- Great Ormside - St James' Church
- [St Mary's church website|http://www.stmarys-lytchett.org.uk/page6.html StMary's Website]
- "St Mary and St Cuthbert, Ankers House".
- LEPER'S SQUINT?
- "History Part Two". St Laurence & All Saints, Eastwood, Essex. 2010. Retrieved 17 January 2011.
- "The hagioscope". St Laurence & All Saints, Eastwood Church. Essex churches. Retrieved 17 January 2011.
- St Andrew and St Bartholomew Church
- Church of The Holy Rood The Holybourne Village Magazine Spring Issue 2009
- What is it? A leper's squint
- THE CHURCH OF ST. JAMES THE LESS, SULGRAVE
- Ridley, Nancy (1966). Portrait of Northumberland. London: Robert Hale. pp. 66–67.
- History of St Oswald and St Oswald's Church, Sowerby
- MR. HARRY GILL Upton (part 1)
- Church of St Nicholas, Elsfield Road, Old Marston
- St Nicholas & St Barnabas Churches Kenilworth