The strike deadline was a minute into Monday in each U.S. time zone. An 11th-hour negotiating session was held with the help of a federal mediator Sunday, but it broke down without achieving any results.
Members of the 12,000-strong union began picketing Monday morning at major studios in the Los Angeles area and outside NBC studio at Rockefeller Center in Manhattan.
The first casualties of the walkout are likely to be talk shows, soap operas, and comedy programs that rely on fresh scripts.
Major motion picture studios and television programs typically have stockpiles of scripts that can insulate them from feeling the effects of the strike for a year or longer.
Writers want a greater share of residual profits from television series sold on DVDs and money made from programs shown on the Internet, cellular phones, and other new media outlets.
Producers acknowledge that online viewing is increasing and promise to study the issue, but argue that it is too early to say how profitable it will be.