This study examines structural patterns in plays by Pinero, Shaw, O'Casey, Eliot, Arden and Pinter, and proposes that there are, in broad terms, two types of play structure - "enclosed" and "open" - which may be usefully differentiated for this purpose. The first type is characterised by its precise articulation of the action within a regularly-shaped, often symmetrical framework, and the second by its juxtaposition of strands of action or thematically illustrative episodes within a comparatively loose framework. It is contended that a polarity between those types of structure is demonstrable in modern English drama and has been a significant factor in its development. Part One of the study examines Pinero's social dramas and Shaw's disquisitory plays, analysing the contrasting methods of theatrical patterning used by these writers and discussing why they used these methods. Shaw's opposition to the deterministic effect of Pinero's "well-made" structures is emphasised, and his development of a much more fluid technique whereby the play was allowed "to write and shape itself" is considered in some detail. Part Two of the study considers later modifications and developments of the structural approaches exemplified in the plays of Pinero and Shaw. In the first chapter of Part Two, plays by O'Casey and Arden are analysed to show how these writers have used "open" structure techniques, comparable to those of Shaw, to express broadly similar points of view. In the other chapter to Part Two, plays by Eliot and Pinter are analysed to show the contribution of these writers to the development of a new type of "enclosed" play, as precisely articulated as Pinero's but without the explicit causality of the "well-made" play. It is suggested that this type of structure, while basically "enclosed" in character, represents a partial synthesis of "open" and "enclosed" form.