The quantitative assessment of immunosuppressant drug levels is still one of the most challenging therapeutic drug monitoring procedures in clinical routine. During the past years, several technical developments matured to useable methods. In addition to immunoassays, liquid chromatography- tandem mass spectrometry has become a key method in immunosuppressant therapeutic drug monitoring. This overview should aid in understanding the advantages and disadvantages of the various assays and methods. UKNEQAS proficiency testing results are used for an inter-assay comparison approach.