Lab on Chip devices have a great potential for medical applications as illustrated in three examples. First, a disposable, prefilled capillary electrophoresis chip for monitoring lithium in blood of manic depressive patients is discussed. In a second example, we present a simple chip for counting sperm cells in semen is presented that is used to determine male fertility. Finally, we present the realization of a blood-brain barrier on chip that establishes a versatile platform for drug screening and has the potential to greatly reduce and replace animal testing.