Series are generally defined as groups of films with self-contained stories that share the same principal character or characters and often the same situations and settings. Series may be conceived as such from the outset, as was the case with The Hazards of Helen (119 episodes from 1914 to 1917), or, as in the case of the James Bond (over 20 films from 1962 to the present) and Halloween (8 films between 1978 and 2002) films, they may emerge, evolve, or become institutionalized over the course of many years. Although films in each type of series can be said to constitute episodes, "episode" as a term is probably associated more with serials and preconceived series than it is with open-ended or evolving ones.
Building on precedents established in the mass-circulation press and in popular fiction in the nineteenth and early twentieth centuries, preconceived film series first emerged in the United States with the Edison Company's Happy Hooligan films in 1900 and 1901. In comic or in melodramatic mode, they became firmly established as a trend in the United States and France later in the decade, with the production of Biograph's Mr. and Mrs. Jones films (1907–1908), Kalem's Girl Spy films (1909), and Yankee's Girl Detective films (1910) on the one hand, and Pathé's Boireau (1906–1909) and Nick Carter films (1908–1909), and Gaumont's Romeo (1907–1908) and Bébé films (1910–1912) on the other. While the move toward multireel films in the early 1910s resulted in the emergence of melodramatic serials such as The Adventures of Kathlyn (1913–1914) and of serial-series hybrids such as What Happened to Mary? (1912) and Fantômas (1913–1914), comedy series in one-reel and two-reel form continued to be made. These films were built around comic personalities, such as Roscoe Arbuckle (1887–1933) in the Fatty series (1913–1917) and Max Linder (1883-1925) in the Max series (1910–1917), and animated characters such as Coco the Clown and Felix the Cat.
Serials and features became the norm as far as melodramatic adventure was concerned, but comic shorts featuring the likes of Laurel and Hardy, the Three Stooges, Mickey Mouse, Bugs Bunny, and Daffy Duck continued to be made in series form in the United States for over forty years, shown alongside feature films and newsreels as an integral part of most cinema programs.
During the 1930s and 1940s in particular, B movies, too, became part of these programs. Whether made by small-scale independents like Monogram or Republic, minor studios like Columbia or Universal, or major studios like MGM and Twentieth Century Fox, the majority of B movies were produced in series. These included westerns such as the Hopalong Cassidy films (1935–1944 and 1947–1949), detective and mystery series such as Boston Blackie (1941–1949), The Falcon (1941–1949), The Saint (1938–1954), and Mr. Moto (1937–1939), medical dramas such as Dr. Kildare (1937–1947), and comedies such as Andy Hardy (1937–1958), Henry Aldrich (1939–1944), and Maisie (1939–1947). Series of A films, by contrast, were rare. Examples include Paramount's Road pictures (such as Road to Morocco) with Bob Hope and Bing Crosby (1940–1952) and RKO's Topper films (1937–1941), neither of which were envisaged as a series initially.
In the United States, B series disappeared, along with B movies themselves, in the 1950s, when series programming and series production became a feature of broadcast TV. During the 1960s and 1970s, series tended to evolve on the basis of follow-ups, sequels, and prequels, as in the case of the Planet of the Apes and Herbie films, as well as the Pink Panther and Dirty Harry films. At the same time, a number of western and comedy series produced in Europe and a number of martial arts films produced in Taiwan and Hong Kong were highly successful. Since then, series in the United States have continued to evolve in much the same way, often around blockbuster films such as Superman and Batman (1989), but sometimes, too, around low- or medium-budget horror films (Friday the 13th, Nightmare on Elm Street) and comedies (Police Academy).