Is there anything improv can’t do?

Besides helping produce stars like Stephen Colbert and Tina Fey, improv theaters now run business workshops and train the kind of actors who once would have gone to Juilliard or Yale. For the first time, the hosts of both “The Tonight Show” and “Late Night” have an improv background.

The origins of the current improv boom can be traced to 1999, when the Upright Citizens Brigade opened its first theater. Back then, there were no permanent stages devoted to improvisational comedy in New York. Now there are three. With a bustling school (2,500 students enrolled) helping to underwrite its prolific productions, the Brigade, whose alumni include the stars of Comedy Central’s new sketch show “Broad City” and Sasheer Zamata, the newest cast member of “Saturday Night Live,” has become the most influential name in improv today.

At the start, you needed to take a class to learn the method. No longer. After seven years of work, the troupe has codified its aesthetic and released “The Upright Citizens Brigade Comedy Improvisation Manual.” Far from the first improv guide, it is, however, the most practical, concrete and unapologetically prescriptive. The manual offers insights and helps aspiring comedians, but its authoritative approach has also drawn criticism.