Theoria is an album by Barry Guy and the London Jazz Composers' Orchestra, with pianist Irène Schweizer as soloist.

In the score, the starting and ending points for soloists are precisely demarcated, allowing the musicians a considerable amount of freedom within a fixed structure.

"[7] In a review for AllMusic, Thom Jurek wrote: "This may be Guy's masterpiece in that here he has reached, finally, the perfect balance of incorporating all of his obsessions into one work: classical music, particularly the Romantic period, free jazz, new music, big band swing, blues, and soundtrack scores...

It is beyond language to interpret its effect, but its organization is so complex, so righteously taut with ideas and combinations of harmonic and contrapuntal interconnectedness it just boggles the mind even when looking at the score.

"[1] The authors of The Penguin Guide to Jazz Recordings wrote: "Guy attempts not to juxtapose blandly different styles of playing, but to overlap them creatively, creating diffraction patterns and points of maximum energy.