Alyson Hannigan

The fine art of the musical episode.

The 100th episode of How I Met Your Mother, which airs Monday night, will feature at least one musical number. The series thus joins a select group of TV shows, from Buffy the Vampire Slayer to 7th Heaven, that have turned out a tuner.

A series like Glee is built around a high-school choir, so viewers expect to see singing and dancing. Animated programs, with their talking dogs and boys that come back from the dead, can mount their own tuneful versions of The Music Man or 101 Dalmatians without viewers batting an eye. (They also don’t have to pay for back-up dancers and extra rehearsal time.) But when a show like How I Met Your Mother decides to belt out a number, its writers and directors have to explain why familiar, unmusical characters are suddenly bursting into song.

Some shows settle for preposterous pretexts: In That’s So Raven, the title character convinces her classmates that a scout for the TV talent show Undercover Superstar is visiting their school, which in turn leads to several impromptu production numbers. In the episode "The Bitter Suite," the characters of Xena: Warrior Princess are magically transported to the all-singing, all-dancing land of Illusia. Some series, however, have put a bit more thought into making their musical performances feel more organic. Click here for a video slide show on television’s best musical episodes and how they were justified.