Most flow cytometers collect between 3 and 10 "parameters" or "variables". Each variable can be used to discriminate some component of the cell populations. The standard for data collection has been known as the "Flow Cytometry Standard" or FCS data file structure. Data can be collected in either histogram mode or listmode. It is now routine to collect flow cytometry data in listmode because there are few constraints in the size of data collection files. Originally, flow cytometers did not have good network capability, small hard disks (5-20 Mb, and floppy drive that could save only about 1 Mb of data). With high capacity hard disks and Ethernet access now routine, listmode is the most useful collection modality. Data can be collected in a gated or ungated mode. If the data are ungated, all events registered by the instruments electronic circuits will be saved in listmode files. If gated collection is chosen, it is possible to collect a listmode file of only those events that satisfy certain criteria established by the operator.