Serial (radio and television) partially coincident with television series
Soap operas
The television serial format as known today originated in radio, in the form of children's adventure shows and daily 15-minute programs known as soap operas (so-called because many of these shows were sponsored by soap companies, such as Colgate-Palmolive and Procter & Gamble). Soap operas were specifically engineered to appeal to women (with the intention of increasing sale of soap). They usually ran from Monday through Friday at the same time every day. A show called The Smith Family which ran only one night a week on WENR in Chicago during the early 1930s was credited as the "great-granddaddy of the soap operas" by radio historian Francis Chase, Jr. One of the other shows that helped pioneer the daytime soap opera/serial was The Guiding Light, which debuted on NBC radio in 1937, and then switched to CBS Television in 1952. The Guiding Light's final episode aired on September 18, 2009, having a total of 15,762 episodes air on CBS. Some of the characters in soap operas have been portrayed as long-suffering (a common theme even in some of today's serials along with the social and economical issues of the day). 
Children's adventure serials were more like film serials, with continuing characters involved in exploits with episodes that often ended in a cliffhanger situation; Westerns were a particularly popular format for children's serials on the radio.
Guiding Light and such other daytime television program serials such as Search for Tomorrow, Love of Life, The Secret Storm, As the World Turns, The Edge of Night, The Doctors, Another World, Dark Shadows, One Life to Live, and All My Children were popular in the Golden and Silver Ages of television and still are today.
Aside from the social issues, the style and presentation of these shows have changed. Whereas in the 1950s and 1960s the drama was underscored with traditional organ music, and in the 1970s and the 1980s a full orchestra provided the score, the daytime dramas of today use cutting-edged synth-driven music (in a way, music for soaps has come full-circle, from the keyboard to the keyboard).
The nighttime serials are a different story, though the concept is also nothing new. In the 1960s, ABC aired the first real breakthrough nighttime serial, Peyton Place, inspired by the novel and theatrical film of the same name. After its cancellation, the format went somewhat dormant until Norman Lear produced Mary Hartman, Mary Hartman in 1976. In 1977, ABC created another comedy soap (aptly called Soap). Although the show was controversial for its time (with a homosexual character among its cast roster), it was (and still is today) a cult classic.
The success of Dallas popularized serial storylines on prime-time television. Its end-of-season cliffhangers, such as "Who shot J. R.?" and "Bobby in the Shower?", influenced other shows like Dynasty (ABC's answer to Dallas), Knots Landing, Falcon Crest, The Colbys, Flamingo Road, Hotel, The Yellow Rose, Bare Essence, and Berrenger's. There were some serial shows such as Hill Street Blues and St. Elsewhere that did not officially fit into this category, but were nonetheless ratings hits season after season.
While the last of the 1980s nighttime soaps ended during the first years of the following decade, then a second wave came with series like Beverly Hills, 90210, Melrose Place, Models, Inc., Savannah and Central Park West. But as the 1990s came to a close, the primetime soap as an official format gradually faded away, where it largely seems to remain as of the middle of the first decade of the 21st century in the United States.
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