In comic books and other stories with a long history, first appearance refers to the first occurrence to feature a fictional character.
Monetary value of first appearance issues
First appearances of popular characters are among the most valuable comic books in existence. In their spring 2002 issue, the editors of The Overstreet Comic Book Price Guide listed the ten most valuable comic books and seven were first appearances of popular superheroes. (Another, Marvel Comics #1 (Oct. 1939), is the first appearance of the Golden Age Human Torch but it is probably more noteworthy because it was the first comic book published by industry giant Marvel Comics).
By the time a character is well-known; even iconic, many years have passed since his or her first appearance and few copies, and fewer good-conditioned copies, remain. These comic books may be worth thousands of dollars. In 2004, a copy of Flash Comics #1 (Jan. 1940), the first appearance of The Flash, was auctioned for $42,000  and a copy of Captain America Comics #1 (March 1941), the first appearance of Captain America sold for $64,400 . In 2010, another copy of Flash Comics #1 sold privately for $450,000.
In February 2010 the first appearance of Superman was auctioned by the creator of the comic book grading system for $1,000,000. It was called the holy grail of comic books as not only was it the first appearance of Superman, it was also one of only two copies known to exist in the 8.0 grade. This issue essentially ushered in the Golden Age of Comic Books and began the superhero genre. Fewer than a hundred copies are known to exist. Within a few short days the first appearance of Batman was sold in a Heritage auction for $1,075,000. The previous record for a comic book sale was set in 2009 at $317,000.
- The importance of the character(s) that debuted; the first appearance of Spider-Man in very fine condition is listed at $45,150; the first appearance of the less popular Iron Man, in the same condition, is listed at $3,837.
- The rarity of comic book itself; comics from the Golden Age are usually more valuable than later comic books because they are older and fewer copies survive. Spider-Man is undoubtedly more popular than The Spectre but Spider-Man’s 1962 first appearance is valued at $45,150 while a copy of The Spectre's 1940 debut, in fine condition, is valued at $54,000. Also, first appearances often lack value if they are relatively recent issues of high-profile, best-selling titles. Except during a 1990s collector’s bubble, the first appearances of several Image Comics characters and newer X-Men have not been as valuable as one may expect for such popular characters because those comics were widely produced.
- Other reasons for historical importance; The Fantastic Four (Nov. 1961) #1 is not only the first appearance of the eponymous group but also represents a turning point in the history of Marvel Comics and is the a first issue of a long-running series.
- Occasionally, a comic book is the first appearance of more than one important character. Usually the characters are related; X-Men #1 (Sept. 1963) introduced the X-Men and their archenemy Magneto. However, rarely a comic book is the first appearance of two unrelated, important characters. More Fun Comics #73 (Nov. 1941) introduced both Green Arrow and Aquaman, who have little relation to one another . This is also the case with Action Comics #1, which contained the first appearances of Zatara and Tex Thomson, as well as Superman.
- Occasionally a first appearance will lack the value expected for a character of such stature because the debut was not splashy. Wonder Woman, an immensely popular and historically important hero, debuted in All Star Comics #8 (Dec. 1941) in one of several stories and was not featured on the cover. This issue is valued at $30,000 in fine condition. Comparatively, the first appearances of equally (or even less) important peers Green Lantern and The Flash, boldly introduced on their covers, are worth $131,250 and $69,000, respectively. Arguably, the first appearance of Wonder Woman is worth much less because she did not make a flashy debut that lent the comic book an air of history.
- As is the case with all collectibles, condition greatly affects the value of comic books, although considerable wear is expected for decades-old comics. Most comic books are worth more if their condition is certified and they are protectively packaged (or "slabbed") by the Certified Collectibles Group, a professional grading service involved in the sale of most high-value comic books, although some fans accuse the group of inflating the value of comics .
Reader interest in first appearances
Collectors value first appearances for their rarity and historical value, while many regular readers are interested in viewing how their favorite characters were originally portrayed. Reprints of first appearances are often published, both as single comic books and in trade paperbacks, usually with other early appearances of the character. Marvel Comics' "Essential" line has become popular by giving readers an affordable glimpse into characters' early history .
Historically, first appearances tell the origin story for the character, although some, such as Batman and Green Goblin, remained dubious figures for several issues. Modern writers prefer to tell a character’s origin across an entire story arc or keep a newly introduced character mysterious until a "secret origin" issue. Some fans consider this a gimmick and prefer the older method .
The artistic merit of many first appearances is debatable. The events portrayed in most famous first appearances are continuously retconed, rebooted and/or expanded upon by subsequent writers. Like many golden and silver age comics, first appearances often become dated and do not fit the modern portrayal of the character.
However, some first appearances are considered classics. 1990s-era Spider-Man writer Howard Mackie said that his favorite story featuring the character was his first appearance and origin story in Amazing Fantasy #15 (Aug. 1962), stating that writer Stan Lee and artist Steve Ditko "gave us everything we needed, I wanted or could ask for in the least possible space. Every single person who retells the origin never improves on the original, they simply expand it." 
Ambiguity of first appearance
While a seemingly a simple concept, determining the first appearance may be complex; comic book fans are infamously nitpicky about such matters. The following are instances in which a character’s first appearance may be difficult to determine:
- Those unfamiliar to comics may assume that Iron Man’s first appearance is The Invincible Iron Man #1 (May 1968). However, in the golden and early silver ages of comic books, few superheroes debuted in magazines carrying their names. More often a character first appeared in a generically titled anthology series. If the character proved popular, a new series was launched. For example, Iron Man first appeared in Tales of Suspense #39 (March 1963) and appeared regularly in that series for five years before Marvel launched a series properly named Iron Man. Wonder Woman, Spider-Man, The Mighty Thor and many others also first appeared in anthology series.
- The first appearance of "all-star" teams is given as the first instance in which that team banded together regardless of whether or not it consists of previously existing characters. The first appearance of The Justice League of America is considered The Brave and the Bold #28 (May 1960), the issue in which they first operated as a group, although none of its members first appeared in that issue. Alternatively, X-Men #1 (Sept. 1963) is both the first appearance of the X-Men and that of each of the team’s original members.
- Sometimes a character first appears in the last page of an issue, foreshadowing his or her greater role in the next issue. Arguments can ensue over whether the first appearance is the issue containing the final page cameo or the subsequent issue which more adequately introduced the character. Wolverine was first seen in the last page of The Incredible Hulk #180 (Oct. 1974) but makes a more full appearance in issue #181 (Nov. 1974). Stricter fans may consider The Incredible Hulk #180 Wolverine’s first appearance but most consider it #181. ComicsPriceGuide.com lists a copy of issue #180, rated very fine, at $149 and #181 at $2,075. Comparatively, The Incredible Hulk #179 (Sep. 1974), which has no special importance, is listed at $11, so both types of first appearance add value to a comic book.
- Retconning can also complicate first appearances. Initially, Cable was portrayed as a wholly new character, first appearing in The New Mutants #87 (March 1990). However, writers later changed his background, stating that Cable is an adult, time-traveling Nathan Summers, the son of Cyclops and Madelyne Pryor, first seen in Uncanny X-Men #201 (Jan 1986). Both issues could be given as the first appearance of Cable. Further complicating the matter, Cable was seen in a cameo at the end of The New Mutants #86 (Feb. 1990).
- Some superhero identities are used by more than one character. The original Green Lantern first appeared in All-American Comics #16 (April 1940). During the Silver Age, Green Lantern, like many DC heroes, was rebooted with a totally new identity. The second Green Lantern, Hal Jordan, debuted in Showcase #22 (Oct. 1959). All-American Comics #16 is still considered the first appearance of Green Lantern, both of the original title-bearer and the superhero identity itself. To avoid confusion, Showcase #22 is called the first appearance of Hal Jordan, of Green Lantern II or of the Silver Age Green Lantern.
- Occasionally, a character will appear in the background of a comic book before fully introduced. Spider-Man’s early love interest Liz Allan is first addressed by name in Amazing Spider-Man #4 (Sept. 1963). However, an unnamed character in Amazing Spider-Man #1 (March 1963) is, based on her appearance and dialogue, probably Allan. Plus, Amazing Fantasy #15 (Aug. 1962), shows an unnamed, unspeaking character who looks exactly like Allan. Thus Allan's first appearance may be given as any of the three.
- Some characters appear in more than one continuity. While the first appearance of Nightcrawler is Giant-Size X-Men #1 (May 1975), the first appearance of "Ultimate Nightcrawler" (Nightcrawler in the alternate Ultimate Marvel universe) is Ultimate X-Men #6 (Aug. 2001).
- Sometimes new characters are created for television or film adaptations of a franchise and are later added to the comic book continuity. The Batman adversary Harley Quinn debuted in the 1992 Batman: The Animated Series episode Joker's Favor. Her first appearance in comic format was the graphic novel The Batman Adventures #12, which took place in the continuity of Batman: The Animated Series. Her first appearance in the regular "DC Universe" was the 1999 one-shot Batman: Harley Quinn. Thus her first appearance is technically Joker's Favor, her first appearance in a comic book was The Batman Adventures #12 and her first appearance in the regular DC Comics continuity was Batman: Harley Quinn. Similarly, Firestar first appeared in Spider-Man and His Amazing Friends #1, which adapted the first episode of the TV series. Her first Earth-616 appearance was in The Uncanny X-Men #193.
- Rarely, a character debuts in a publisher’s foreign branch and then appears in a domestic series. Psylocke first appeared in Captain Britain #8 (Dec. 1976), an original series of Marvel UK not widely available outside of Great Britain. Her debut in an American series was The New Mutants Annual #2 (1986). Her first appearance is sometimes given as either but more correctly it is Captain Britain #8 while The New Mutants Annual #2 is her first US appearance.