A tibial milling guide which incorporates a combination of instruments for producing a planar surface on a portion of the tibia using a milling device. The system includes: an extramedullary alignment guide with a detachable reference guide, an adjustable milling base assembly, a detachable milling template, a sizing gauge, and a milling depth gauge. The reference guide is used to position the alignment guide to the tibia. The reference guide has a T-shaped member which provides a planar reference and a centering reference to the tibial head. The reference guide also allows the surgeon to set the rotational alignment before the alignment guide is secured in place. The adjustable milling base assembly is also detachably connected to the alignment guide. The milling base assembly includes a slide and a shiftable base that allows for lateral adjustments of the template with respect to the articular surface after the alignment guide has been properly secured to the tibia. The milling base is also extensibly connected to the alignment guide to allow the spacing between the articular surface and the template to be adjusted.