Like most playwrights of his period, William Shakespeare did not always write alone. A number of his surviving plays are collaborative, or were revised by others after their original composition, although the exact number is open to debate. Some of the following attributions, such as The Two Noble Kinsmen, have well-attested contemporary documentation; others, such as Titus Andronicus, are dependent on linguistic analysis by modern scholars; recent work on computer analysis of textual style (word use, word and phrase patterns) has given reason to believe that parts of some of the plays ascribed to Shakespeare are actually by other writers.
In some cases the identity of the collaborator is known; in other cases there is a scholarly consensus; in others it is unknown or disputed. These debates are the province of Shakespeare attribution studies. Most collaborations occurred at the very beginning and the very end of Shakespeare's career.