A One-Name Study (ONS) is a project researching all occurrences of a surname, as opposed to a particular pedigree (ancestors of one person) or descendancy (descendants of one person or couple).

A one-name study may concentrate on aspects such as geographical distribution of the name and the changes in that distribution over the centuries, or it may attempt to reconstruct the genealogy of the lines bearing the surname. A frequent aim is to identify a single original location of the name, especially if the name appears to derive from a place name. (But for many names, for example those indicating an occupation like Butcher, or a patronymic-type surname such as Peterson, there will not be a single origin.) Some one-namers also run an associated DNA surname project to assist the analysis of origins.

The objective of a One-Name Study is not just the collection of data; collection is a means to an end. The One-Name Study aims to research the genealogy and family history of all persons with a given surname (and its variants). In doing this, it attempts to ascertain, amongst other things,

• The origin of the name or early references
• The name's meaning: is it patronymic, topographical, toponymic, occupational, etc? Or a mix of these?
• Relative frequency
• Distribution in geography and time
• Patterns of immigration and emigration
• Name variants and "deviants" (see Variants and deviants)
• Reconstructing all families, that is, assembling all the people with the given surname into pedigrees, is not a required part of a One-Name Study, although some researchers who choose a reasonably rare name are able to do this. This is particularly true for UK families, where almost complete vital records for the 19th and 20th centuries are available, as well as censuses for the period 1841-1911. It is important that a Guild registered One-Name Study takes a worldwide approach, but the difference in ready accessibility of data across countries will temper what it is possible to achieve.

Some "One-Namers" may restrict their research geographically, perhaps to one country, but true One-Namers collect all occurrences world-wide. The Guild of One-Name Studies accepts only true one-name studies for registration, although anyone with an interest in one-name studies is welcome to become a member.

There is much scope for co-operation between one-namers and other genealogists and family historians, and many one-namers are also engaged in more conventional pedigree hunting of their other ancestors. Many drift into a one-name study as a way of eliminating alternatives when researching a particular ancestral name. A co-operative effort between people studying the same surname bears much fruit, and they have a good chance of discovering new relatives, depending of course on how common the name is.