Invader Zim was first broadcast on March 30, 2001 and was targeted at children in their early teens, and met with critical acclaim, but after the first season, the show's ratings began to suffer. Before the second season was completed, Nickelodeon canceled the series, leaving at least 23 episodes and a planned television movie series finale unfinished. The show has maintained a cult following since cancellation with a pilot episode, 46 canon episodes, and at least 17 episodes that were in production at the time of the series' cancellation. On August 19, 2006, all 46 canon episodes have aired on television and been released on DVD. 19 of the episodes are split into two individual stories, each lasting about 12 minutes. There are 8 double-length episodes. Almost every one was directed by Steve Ressel; the only exception is the pilot episode (directed by Jordan Reichek).
Invader Zim revolves around the life of its lead character, Zim, his malfunctioning robot GIR GIR and the humans and aliens who become involved in his schemes. Zim is a member of the imperialistic Irken race, an extraterrestrial species whose social hierarchy is based on height. The Irken leaders, known as The Almighty Tallest, serve together because they are exactly the same height. As recounted during the series premiere, "The Nightmare Begins", Zim, who is both very short and very naïve, was banished to the planet Foodcourtia for overzealously attacking his own planet and allies during Operation Impending Doom I. Upon hearing of Operation Impending Doom II, Zim leaves Foodcourtia and arrives at the Great Assigning. Here, elite Irken soldiers are chosen to become Invaders: special soldiers whose mission is to blend in with the indigenous life forms of their assigned planet, collect intelligence and prepare the planet for conquest by the Irken Armada. Zim pleads with the Tallest to assign him a planet and, in a desperate act to get Zim as far away as possible, they assign him to a presumed planet on the outskirts of their known universe that has never been explored. This "mystery planet" is Earth, and Zim's erratic attempts at world domination and/or the eradication of the human race begin when he lands on Earth for the first time.
- Zim (Richard Steven Horvitz): Overzealous, impulsive and convinced of his own greatness, Zim is incompetent as an Invader. He constantly schemes to enslave or destroy mankind and, despite his access to potent Irken technology, his plans, frequently based on misconceptions, are never successful.
- GIR (Rosearik Rikki Simons): Zim's only companion, GIR is a SIR (Standard Information Retrieval) unit, an Irken standard-issue robot. He was made out of some scraps in a trash bin by The Almighty Tallest. The name "GIR" is a self-applied moniker that is never explained; GIR himself states that he doesn't know why he refers to himself as GIR. GIR differs physically from other SIR units in that his head contains odds-and-ends rather than artificial intelligence hardware. Because of this, GIR displays a wildly erratic personality. He is almost completely irreverent and rarely obeys Zim's commands, variously due to distraction, disinterest and disobedience. GIR disguises himself by wearing a green dog suit which fools most humans, despite looking almost nothing like an Earth dog.
- Dib (Andy Berman): Zim's main opponent on Earth. He has been obsessed with the paranormal and supernatural ever since he was a baby and instantly recognizes Zim as an alien, but everyone derides him as "crazy" or "insane", including his sister and father.
- Gaz (Melissa Fahn): Gaz is Dib's cynical, humorlessly sarcastic and psychotic younger sister. She plays a prominent role in the show, but rarely takes an active part in attempting to hinder Zim's schemes. Though she knows and admits Zim is an alien, she believes Zim is too dumb to actually be a threat. She enjoys a very limited number of things, a few of them being pizza and video games.
- Almighty Tallest (Wally Wingert; Kevin McDonald): Red and Purple, collectively known as the Almighty Tallest, are the supreme Emperors of the Irken Empire. They are united in their love of destruction, tormenting of others, snacks, puppet shows and their hatred of Zim. Due to their own incompetence, it appears the only reason for their ruling status is their unusually tall height.
The main cast is supported by recurring characters (Dib's teacher, Ms. Bitters, and Dib's father, Professor Membrane). Later in the series, Sizz-Lorr and Tak are extra antagonists for Zim. Lard Nar and his resistance group, The Resisty, are the enemies of the Irken Empire.
Invader Zim was created by comic book writer Jhonen Vasquez who indicated that very little of his writing style was changed, other than restricting language and certain visuals that may not be suitable for children. The biggest difference that he cites is the change from working alone to working with thousands of people at Nickelodeon, calling it an "absolute misery". Apart from a few double-length episodes (including the debut), each episode is a self-contained storyline. Sometimes the occurrences in previous episodes are mentioned but there is no continuing storyline other than the main invasion plot. According to the DVD commentaries, Invader Zim takes place in an alternate universe of its own, with a present-day Earth more technologically advanced than the real Earth. The most common setting of the show is where Zim spends most of his time "infiltrating" the human race at a school (spelled "Skool" in the show), or in his home planning his next "big" takeover. However, towards the end of the show, a larger universe of characters and organizations were beginning to build continuity: Dib's capture and repair of a downed spaceship and use of it to learn more about Zim's race; a resistance to the Irken Empire (The Resisty); an Irken seeking revenge against Zim (Tak); and various other devices which were becoming more and more a part of the show's central mythology.
A full first season and half of a second season of Invader Zim were produced before Nickelodeon cancelled the show due to low ratings among the channel's target audience and the high cost of making the show.  In season two, the animation style became slightly more stylized and pronounced in motion than season one. The majority of the completed season two episodes were never aired on Nickelodeon in the North American territories, although they were all eventually aired on Nicktoons Network in mid-2006.
Since November 25, 2006, both seasons of Invader Zim are available for download on the Xbox Live Marketplace, Zune Marketplace, and PlayStation Store. In addition, the majority of the show's episodes are available for download on iTunes.Episodes are also available for streaming on Netflix. On March 2010, episodes of Invader Zim were aired on Nicktoons. The reruns were the second highest-rated show on the network, and according to Vasquez, were part of a plan by the network to see if a revival of Invader Zim was feasible. Vasquez also asserts that, despite widespread rumor, he would have returned to the show again had Nickelodeon not deemed the revival "too expensive".  On December 24, 2011 the series pilot had its television premiere followed by the result of Nicktoons' "Girreatest Zim Moments" online poll.
Vasquez asked his friend, Mark Tortorici, to produce ideas for the theme music for Invader Zim. The two settled on a direction for the music and Tortorici and produced a final version of the theme. Vasquez's team selected Michael Tavera to compose music for the pilot episode. Vasquez said that he asked for several of the tracks that Tavera produced to be discarded and that ultimately the team had less than half of the number of tracks that they planned to submit. According to Vasquez, he and Tavera were not creatively "a great fit" for one another. Vasquez described the pilot music as having a "more 'children's television' sound", with a "much more traditional and not as surprising" theme as he wanted, but added that the music "worked for the pilot". Tortorici's theme song and Tavera's cover version did not appear in the pilot as there was no credit sequence. Vasquez said that members of the Invader Zim crew laughed at Tavera's version of the theme because it was "cheesy", adding that the reaction was not mean-spirited and that Tavera had received little information about the series before submitting the music. Tavera did not become a part of the regular Invader Zim crew, while Kevin Manthei created the series' music.
Invader Zim was critically acclaimed, winning two awards and nominations for the first episode. The DVD release has also received high scores and positive acclaim.
Awards and nominations
During its initial broadcast, Invader Zim was the recipient of three awards and seven nominations. In 2001, Kyle Menke won an Emmy for Outstanding Individual Achievement in Animation for "The Nightmare Begins" for his storyboarding, as did Steve Ressel, who won an Annie for Outstanding Individual Achievement for Storyboarding in an Animated Television Production for "The Nightmare Begins". That same year, Steve Ressel, Jhonen Vasquez, and Mary Harrington won the World Animation Celebration award for Best Title Sequence. The show was nominated for five other Annie Awards in 2001: Outstanding Achievement in a Primetime or Late Night Animated Television Production, Outstanding Individual Achievement for Directing in an Animated Television Production (Steve Ressel for episode "Dark Harvest"), Outstanding Individual Achievement for Music Score in an Animated Television Production (Kevin Manthei), Outstanding Individual Achievement for Production Design in an Animated Television Production (Vasquez), and Outstanding Individual Achievement for Voice Acting by a Male Performer in an Animated Television Production (Richard Steven Horvitz). In 2002, the sound crew was nominated for the Golden Reel Award for Best Sound Editing in Television - Animation for "The Nightmare Begins". In 2003, the show was nominated post-cancellation for an Annie for Outstanding Achievement in an Animated Television Production.
Criticism and controversy
Negative criticism came when Invader Zim was cited in the murder trial of Scott Dyleski in 2006. The prosecution asserted that the defendant had a fascination with images of body parts. They drew attention to comments he had made after watching the Invader Zimepisode "Dark Harvest", in which Zim collects human organs in an attempt to appear more human. Witnesses for the defense said that the comments were made in jest. Copyright: http://en.wikipedia.org/wiki/Invader_ZIM