Used to enlarge a pilot hole to the final wellbore diameter or to condition the wellbore to gauge (smooth the wall, remove ledges and doglegs).

· Fixed Hole Openers: PDC or TCI cutters on a fixed body.
· Hydraulic / Expandable Hole Openers: Cutters extend by hydraulic pressure; retract for tripping, enabling under-reaming below casing.
· String Reamers: Placed within the BHA to maintain gauge through abrasive formations.
· Near-Bit Reamers: Placed directly behind the bit.