Harryhausen model, Medusa, from the Clash of the Titans.
It is hard to imagine in this age of CGI animation seen in movies, but there was a time in the not-to-distant past when animation was done in stop-motion photography. Some of this was done with clay- Claymation as they call it. Ray Harryhausen usually worked alone in the studio building, moving, and filming his created creatures.
Harryhausen got his start shortly after the movie, King Kong (1933) came out. The artist in that movie, Willis O'Brien, was his mentor and influencer.
Says Harryhausen, "I first saw King Kong by accident," he has said, "My aunt was a nurse and had been taking care of Sid Grauman’s mother. She was given three tickets for the film and took my mother and myself. I haven’t been the same since." 
Ray began experimenting with stop-motion with a friend's camera and with help of his father, created a studio at the back of his garage (Ever notice how many brilliant things come out of a garage? Apple Computers and Google, for instance)
Ray Harryhausen with two of his stop-motion sculptures from the movie, "Clash of the Titans."
Soon he was working on his first film, Evolution. Later he animated sequences of film educating soldiers during World War II. Other well-known movies in which the art of Harryhausen appears are War of the Worlds (1948), Mighty Joe Young (1949), The Tortoise and the Hare (1952- later finished in 2000), The Beast from 20,000 Fathoms (1953), The 7th Voyage of Sinbad (1958), It Came from Beneath the Sea, Earth vs. the Flying Saucers, The Three Worlds of Gulliver (1960), Mysterious Island (1961), Jason and the Argonauts (1963), One Million Years B.C. (1967), and Clash of the Titans (1981).
Ray created his models using a variety of ingredients. To create the skeleton on the right, he started with a 10 inch metal armature and added joints and sockets. He then dipped cotton in liquid latex and applied it to the armature. To animate a head, plaster casts were made with a variety of expressions that could be interchanged as filming progressed. Clay was also used to mold foam latex figures.
Animated skeleton from Jason and the Argonauts
Stop-motion is still used at times along with pixelation and CGI. RSA Films has been working on a film with a mixture of the two. Before CGI, a more recent stop-motion movie was Rudolph the Red Nose Reindeer. Gumby, an excellent example of claymation, was watched by millions of baby-boomers.
Many animators and modelers admire Harryhausen to this day. Ray appeared in numerous horror and scifi conferences. When he autographed pictures, the line was usually very long. He has earned the title of the father of stop-motion animation.
UPDATE (July 5, 2013) - Sadly, Harryhausen passed away on May 7, 2013. His name remains as a landmark in the history of a genre and cinematic art, the art of dimensional stop-motion animation.
Below is a video sample of Harryhausen's movies:
Below are links of articles relating to stop-motion and Ray Harryhausen:
You can buy books and DVD's of Harryhausen's art and movies. His movies are great for a general audience and art classroom (See below):
Thank you for sharing! This is a wonderful resource. I look forward to sharing information about Ray and showing the you tube video to my students- they will be even more inspired to make our little claymation movies! January 13, 2009 3:02 PM