With increasing complexity of circuit layout on the die and special packages in which the die are flipped over, failure analysis on the die front side, sometimes, can not solve the problems or is not possible by opening the front side of the package to expose the die front side. This paper discusses fault isolation techniques and procedures used on the back side of the die. The two major back side techniques, back side emission microscopy and back side OBIC (Optical Beam Induced Current), are introduced and applied to solve real problems in failure analysis. A back side decapsulation technique and procedure are also introduced. Last, several examples are given. The results indicated that the success in finding root cause of failure is greatly increased when these techniques are used in addition to the traditional front side analysis approaches.