Stephen Sondheim is unquestionably the most influential figure in musical theater of the last half-century. His acerbic lyrics, angular melodies and world-weary themes have changed the personality of the Broadway show. Outside the rock musical and Lloyd Webber subgenres, there is hardly a musical today whose score doesn’t do homage to Sondheim.

This summer, no fewer than three Sondheim-composed shows can be seen on Berkshire stages, representing distinct stylistic moments in his career. Williamstown Theatre Festival opens its season with an early work in the Broadway song-and-dance mode, A Funny Thing Happened on the Way to the Forum. The student performers of Barrington Stage Company’s Youth Theatre, a professional training program, will present Into the Woods, the sweet and sour melange of fairy tales that explores what happens after happily ever after. And Barrington’s mainstage season opens this week with the gory revenge thriller Sweeney Todd.

Forum is a big, brash musical with a convolutedly madcap plot about a crafty slave, delivered in knockabout style and with a setting in ancient Rome that features a brothel full of sexy courtesans. Many productions emphasize the cheesecake as much as the slapstick. But not Jessica Stone’s at Williamstown, opening later this month, which features an all-male cast.

“The show goes back to the original source material, the comedies of Plautus and the stock characters of the Roman theater,” explains WTF Artistic Associate Justin Waldman. “In those shows, men played all the parts. Going back to the way these shows were originally produced heightens the archetypes of these characters and the clowny spirit of the theater of that time.” It’s no campy drag-fest, though. “It’s not a send-up of the female characters, it’s men playing women for real. And the female characters become more a part of the power plays in the plot.”

Sweeney Todd, the Demon Barber of Fleet Street is as dark as Forum is sunny. The tale of the barber who razors his customers’ throats and whose landlady processes the corpses into tasty meat pies turned the musical theater on its head when it premiered in 1979. Originating as a bloodthirsty Victorian “penny dreadful,” in the Hugh Wheeler/Christopher Bond script the story becomes a revenge tragedy, with the barber wreaking indiscriminate vengeance for wrongs he has suffered. It’s been seen as a psychological study of obsession and as an allegory of rapacious capitalism and the class system.

Sondheim’s score is eclectic and complex, and his lyrics bite with corrosive humor. Where Forum cheerfully promises “tragedy tomorrow [but] comedy tonight,” Sweeney mordantly observes that “We all deserve to die.”

Sweeney Todd: through July 17, Barrington Stage Company, 30 Union St., Pittsfield. Into the Woods: July 14-Aug. 15 , BSC Youth Theatre, Pittsfield and Great Barrington, (413) 236-8888, barringtonstageco.org.

A Funny Thing Happened on the Way to the Forum: June 30-July 11, Williamstown Theatre Festival, Williams College, Williamstown, (413) 597-3400, wtfestival.org.