Protein kinases are enzymes that transfer phosphate groups from high-energy donors (typically nucleoside triphosphates) to surface exposed Serine, Threonine or Tyrosine side chains of other proteins and thus alter their biological properties. There are more than 500 different protein kinases in the human proteome involved in regulatory pathways. They control metabolism, gene expression, hormone response, cell growth and many others. Therefore these Kinases are also involved in emergence or fighting of various diseases such as cancer. For this reason, inhibitors that block particular protein kinases are invaluable tools for the investigation of regulatory pathways and diseases and of course especially for therapy.