Like most playwrights of his period, William Shakespeare did not always write alone.
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.
These debates are the province of Shakespeare attribution studies.
Most collaborations occurred at the very beginning and the very end of Shakespeare's career.
The unscrupulous nature of the Elizabethan book printing trade complicates the attribution of plays further; for example, William Jaggard, who published the First Folio, also published The Passionate Pilgrim by W. Shakespeare, which is mostly the work of other writers.