Success rates after free tissue transplantation (FTT) have greatly improved over the last 20 years, partly due to improved technical performance of microvascular anastomoses with better optical and instrumental aids. However, flap failure is still a clinical problem and occurs in 5 to 10%, mainly due to blood vessel thrombosis within the first 24 postoperative hours. The clinical results after FTT can be optimized by in-time diagnosis of irreversibly compromised tissue blood flow and immediate operative reexploration. Therefore, there is a special demand for adequate and reliable postoperative monitoring techniques. This article reviews all monitoring techniques which have been performed in the experimental-clinical setting after FTT thus far.