Most Welsh surnames are patronymic; that is, they are derived from a personal name of an ancestor. In the Middle Ages, the prefixes ap and ab were commonly found in Welsh surnames. Welsh names used to include strings of Patronymics going back through the generations, until the 16th century when people began to use fixed hereditary surnames. Evidence of these prefixes can still be found in many Welsh surnames, such as Prince, Probert, Bowen, and Beddoes.

As compared with other national and linguistic groups, Welsh surnames are comparatively few in number. Due to this, the use of bynames was a common practice. A byname was linked with a patronym, and could indicate the occupation of the bearer, or perhaps a nickname. An example of this would be Jones the Post. In this case, the patronymic surname is Jones, with the byname, Post, a reference to the person's occupation, being used to differentiate this one person from others of the name Jones. The use of bynames was still common in recent generations, although they rarely appeared in written form.

Welsh surnames rank highly within a list of the ten most common surnames in Britain: Jones in second position, Williams in third; Davies in sixth; Evans in eight; Thomas in ninth; and Roberts in tenth.