As much as modern media likes to paint goth as something novel and out of the ordinary, goth has pretty much always existed, in most cultures; it was just never identified or named as a separate movement before the mid-19th century. It is not a strictly western-European phenomenon (Russian culture, for example, has always been remarkably goth), but the identifying factors and naming conventions have all pretty much come from western Europe.

"Goth" originally referred to Germanic tribes who conquered and controlled much of Europe in the Middle Ages. "Gothic" referred to a style of art and architecture, and was originally a demeaning term used by people during the Renaissance, to indicate their contempt for the "crude" and "unenlightened" culture of the Goths as compared to their own. (A classic goth joke, when asked "Are you gothic?", is to respond, "No, I'm Romanesque." Only goths would find architecture jokes funny.)

The Middle Ages were, in fact, quite gothic. There was a fascination, bordering on obsession, with the contrast between good and evil, with death, and with the struggle between purity and decadence. There was also a great deal of remarkable and striking art and literature on these themes produced during this time (roughly 300-1300 C.E.), and all of this no doubt was a factor in the appreciation the Romantics developed for this period in history.

In the early 19th century, an artistic movement called Romanticism arose. It was focused around fantastical themes, the ongoing struggle between good and evil, sensuality, and frequently death. From this movement arose a smaller movement, personified by writers like Byron and Shelley, that was increasingly morbid and decadent. This more morbid style came to be known as gothic, in part because of the appreciation of its leaders for the "Gothic" style of the Middle Ages.

Victorianism repressed the sensuality of Romanticism and gothic, but kept the good-evil dichotomy and the obsession with death, overlaid with a strict morality that nevertheless kept the darker impulses bubbling and more likely to burst out in bizarre ways.

As the 20th century dawned and technology began to move at dizzying speed, most of Victorianism was discarded (though in America the repression of sexuality would remain a large aspect of the culture). The two World Wars gave people more death and despair than they could ever have wanted, and in their wake a determination arose to make life more purposeful and enjoyable (this compounded by technology and increasing wealth for the working classes).

The identifiably distinct goth subculture we know now began to flower in the early '80s, an offshoot of the British punk scene. It's no coincidence that this was the Thatcher/Reagan era, when the superiority of Anglo-American culture was being forced down the world's throat, when conservatism and conformity and being bright and happy and avoiding anything disturbing were being touted as the only acceptable behaviors, and the economy was in recession, with the difference between rich and poor beginning to grow alarmingly. What began as a reactionary movement found continued life in the ever-increasing anxiety and isolation of our culture, and it continues today.

Goth, the goth "mood," and goth thinking became increasingly visible in the mid 1990s, with the popular success of bands such as Nine Inch Nails, movies such as The Crow and Seven , and TV shows such as "The X-Files" and "Millenium." The new millenium brought goth a level of recognition that verges almost on acceptance; Hot Topics are fixtures in shopping malls and neo-goth-identified bands such as Evanescence and H.I.M. have seen a fair amount of popular succes. Over the past decade or so there has been a cycle of goth periodically being fashionably trendy, with clothing designers and cosmetic companies borrowing goth styles and looks for a season or two. At the same time, the "classic" goth style (in both fashion and music) has waned significantly, with the social/musical scene nowadays focused more around electronic and industrial music and a harder, more technology-oriented sense of style.

Goth is pretty much here to stay. Its ability to evolve and adapt, while still answering the need of a portion of our culture for the dark and perverse, has largely guaranteed that it will stay for a good long time. It's just a matter of what form it takes.

More information:
An Early History of Goth examines the evolution of goth out of the post-punk landscape.
Blood Dance takes a more esoteric look at the origins of goth.