Virus classification is the process of naming viruses and placing them in a 'family tree' in relation to other viruses. This process is different from the one used to classify plants or animals. Thanks to the fossil record, there are clear connections between most animals or plants. Viruses do not leave such a record, so it is more difficult to determine relationships between them. Another factor causing difficulties in virus classification is their pseudo-living nature, many scientists are debating whether viruses should be considered alive because they are missing several criteria considered important for living creatures. This makes viruses very difficult to place in the current classification system for plants and animals.

Virus classification is currently based on five phenotypic characteristics; morphology, or structure, of the virus; type of nucleic acid, or the genetic material, of the virus; mode of replication; hosts; and the type of disease they cause. There are two classification systems in use today, the Baltimore system and the International Committee on the Taxonomy of Viruses classification guidelines.

The Baltimore classification system was developed by Nobel Prize winning biologist, David Baltimore. This system separates viruses into seven groups, designated by Roman numerals, depending on their type of genetic material, the number of strands of genetic material and their method of replication. There are other classification systems that are based on the morphology of the virus or the disease caused. These systems are inadequate due to the fact that some diseases are caused by different viruses, the cold or flu are the most common example of this and some viruses look very similar to one another. Another factor is viral structures are difficult to determine under a microscope thanks to their small size. By classifying viruses based on their genetic material, some indication of how to proceed with research is provided because viruses in a category behave in a similar manner.

The seven groups are: Group I, double stranded DNA viruses such as the herpes virus and the chickenpox virus; Group II, single stranded DNA viruses such as the parvo virus; Group III, double stranded RNA viruses; Group IV, positive-sense single stranded RNA viruses such as the Sars virus, the yellow fever virus and many other well known viruses; Group V, negative-sense single stranded RNA viruses such as the measles virus, the mumps virus and the rabies virus; Group VI, reverse transcribing RNA viruses such as HIV; and Group VII, reverse transcribing DNA viruses such as the hepatitis B virus. The Group VI viruses use the enzyme to reverse-transcribe their RNA into DNA and then insert the transcribed DNA into the host organism's DNA, where it is replicated whenever a cell divides. The Group VII viruses transcribe their DNA into an RNA form, then transcribe the RNA back into DNA to be inserted into the host's DNA and replicated.

In the early 1990s, the International Committee on Taxonomy of Viruses devised and implemented rules for the naming and classification of viruses. This group still oversees the classification of viruses today. The ICTV system shares many features with the system used for classifying cellular organisms. There are several differences however; the classification of cellular organisms starts with the Kingdom, while viruses start with Order; another difference is the species name generally takes the form Disease species. For example the classification of the yellow fever virus is; Family - Flaviviridae, Genus - Flavivirus, Species - Yellow Fever virus. Notice this virus does not have an Order classification; that is because recognition of the Orders has been extremely slow, in over ten years only three Orders have been named. Many of the 80 known families remain unplaced. The ICTV are still working on this aspect of virus classification.