Aisles were first added in the 12th century.  On each side of the nave two low arched openings pierced through the existing walls gave access to the aisles.  These openings were swept away when the present arcade was put in in 1812, but the Norman angle-shafts to the responds can be seen in the wall at each end of the arcade.  In the 15th century wider aisles were built, the Norman south doorway moved out to its present position and a porch was built.  At the east end of each aisle is a squint giving a view of the altar.  One of these is a very rare squint passage which, prior to the installation of choir pews, would have allowed access to the altar.  In the North aisle the west wall contains a 13th century lancet window.  The south wall of this aisle was at first the outside wall of the church and the rough area of plaster above the Anglo-Saxon window is the original exterior plaster of the Anglo-Saxon church.  The wooden box chest is dated 1634.  There are some medieval tiles (13th-14th century) on the floor near it and also a few in the south aisle.

The wooden balcony in the nave is the 15th century rood loft, one of the very few which have survived.  When removed, probably early in the reign of Elizabeth I, it was carefully hidden behind a lath and plaster covering against the east wall of the nave, discovered there in 1812 and since repainted.  The wooden screen below the loft is Victorian.  

The tower is 15th century.  On its floor stands the Norman font (early 12th century).  The Font Figure has long been the subject of speculation and debate.  However, recent research identifies the carving as being that of St Michael, on an original Saxon font, over-cut by Norman carvings and 16th century desecration.  St Michael is also depicted on the font at Winterbourne Monkton, but the Avebury font carving is far earlier than that of Winterbourne Monkton and the saint is depicted as holding a crozier- not a symbol of Episcopal power but rather cosmic, supernatural power.  The carvings show two serpents with twisted tails, their heads turned towards the figure of a bishop or Archangel Michael holding a crozier; popular pictures in the middle-ages showed Christ trampling on the dragons of evil and sin.  It is also worth noting that the tree, carved in one piece, 12 pillars of wood/ tree trunks circling the font contains a wonderful array of birds. 

On the south wall is the funeral hatchment of Lt. General Sir Adam Williamson, KB, of Avebury Manor, appointed Governor of Jamaica 1790 (who died in 1798).  On the north wall are the royal arms of George III, 1760-1820.  The stone coffin is 13th-14th century and is one of three found by the south wall of the chancel, probably those of Priors of the adjacent Benedictine Priory.

The bells are in regular use, restored in 1981.  The oldest bell is the tenor, cast in 1719 by Avebury-born Richard Phelps, master of the famous Whitechapel Bell Foundry 1701-1738.

The chancel and chancel arch are late 13th century.  The choir stalls contain some 17th century woodwork.  The altar rails are early 18th century.  The Bishop’s chair is mid-17th century.  The tablet on the south wall is to John Truslow of Truslow Manor (who died in 1593) – Truslow Manor lies at the west end of the bridle path.  The tablet on the north wall is to Dame Susanna Holford (who died in 1722).  She was the widow of Sir Richard Holford of Avebury Manor and left money for the foundation and endowment of a school in Avebury for children ‘whose parents are not able to teach them to read’.

Set in the east wall of the porch is a fragment of Norman 12th century carved stone.  On the outside of the church tower some large stones can be seen in the wall.  This is an example of Anglo-Saxon side-alternative quoining, being the cornerstones of what was then the north-west corner of the Anglo-Saxon nave.  Here too, built into the wall, is a fragment of Anglo-Saxon cross-shaft of an earlier date.