Inthispapertheconceptofabsoluteobservabilityofnonlinearcontrolsystemsisadvanced.Differentfromthelineartime-invariantversion,therearedifferentdefinitionsofabsoluteobservabilityfornonlinearcontrolsystems.Twoalgorithmsformaximalabsolutelyobservablesubsystemsaregiven.Correspondingly,therearetworelevantnormalforms.Therelationswiththelargestcontrolledinvariantdistributioncontainedinkerdh,zerodynamicsetc.,arediscussedfromtheviewpointofmaximalabsoluteobservabilities.