Sternoclavicular joint instability is an uncommon but challenging clinical problem for the orthopaedic surgeon. Although most cases can be treated nonoperatively with minimal long-term pain or functional limitation, a small percentage may require surgical intervention. This includes chronic anterior instability associated with persistent pain or functional limitation as well as irreducible or recurrent posterior instability. Although numerous procedures have been described for treatment of the unstable sternoclavicular joint, the optimal method for stabilization has not been determined. Here we describe a technique using Achilles tendon allograft to create an interpositional arthroplasty and briefly present 3 illustrative cases in which we performed the procedure. We believe this method effectively stabilizes the joint, restores function, relieves pain, and has applications for traumatic sternoclavicular instability as well as instability related to other etiologies.