They were initially all clones (of Jango Fett), but to them other clones and enlisted humans - soon considered largely inferior - were later added.
See more in the Wookieepedia article .
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While the initial troopers were all clones of Jango Fett, after the installation of the Galactic Empire most of the men in the white armor were either enlisted or conscripted humans of normal birth.
This is evident by the varying heights, sizes, and voices of the Stormtroopers seen in the original trilogy.
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In Allegiance , stormtrooper protagonists suggest that the Emperor conciously stops producing more clones, favoring regular humans.
I guess that while he would appreciate clones as useful tools, he would still consider them inferior to "pure" humans.
He is a hopeless racist, after all.
Also, he has no need of them anymore.
Jango Fett was a good template, but the main advantages of the clone army was short breeding and training cycles and absolute obedience.
Once the Empire is firmly in place, the Emperor can take all the time he needs to train and brainwash regular troops.
He has a vast pool of recruits, too, as opposed to when he was Chancellor.
Furthermore, I seem to remember that the clones did not fare too well when living longer.
They were made to fight hard and die quickly, not to serve for decades.
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Given that Luke and Han could both wear stormtrooper uniforms, and they fit reasonably well
, I'd say that they can't all be clones by the time of episode IV -- there's about five inches of difference in their heights.
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No, they are not all clones, if even some should be.
The sources that I know of are in the EU only, but there are several references in the books that points to both Han Solo and Kyle Katarn attending Imperial Academies for stormtrooper training.
The references are very "natural" with no mention of any clones, suggesting that such reqruitment/training facilities were quite common so that most (if not all) stormtroppers would be ordinary men who enlisted for various regions.
In the films, they are not all of same height and have different voices, suggesting again that they are not clones.
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Clones were phased out of the Stormtrooper Corps shortly after the end of the Clone Wars, and were replaced with non-clone volunteers.
Out-of-universe, Pablo Hidalgo has confirmed that the stormtroopers are not clones as of the events in Star Wars Rebels (which occur 5 years before the events of Episode IV).
The explanation given is that the production of clones has been phased out, and since the clones age at twice the rate of normal humans they are too old to serve as stormtroopers.
They have been replaced with non-clone volunteers who are patriotic and loyal to the Empire.
Video evidence can be found on Youtube (starting at about 2:56 into the video) .
In-universe, the canon novel Tarkin confirms this fact since it includes an incident where a group of stormtroopers are seen by Moff Tarkin without their helmets.
The stormtroopers are led by a Kamino clone sergeant but all the other troopers are non-clone recruits.
Here is the relevant quote from p. 94
(the stormtroopers are loading Darth Vader's meditation chamber onto Tarkin's ship): When the stormtrooper operating the equipment accidentally allowed the flattened sphere to bang against the edge of the cargo hold’s retracted hatch, Vader stamped forward with his gloved hands clenched.
“I warned you to be careful!”
he shouted up at the trooper.
“My apologies, Lord Vader.
Wind shear from—”
“Excuses won’t suffice, Sergeant Crest,” Vader cut him off.
“Perhaps you are aging too quickly to remain on active duty.”
Tarkin couldn’t make sense of the remark until he realized that Crest’s was a face he had seen countless times during the war—the face of an original Kamino clone trooper.
The bare-headed others comprising Vader’s squad were human regulars who had enlisted after the war.
The novel takes place about 5 years after the end of the Clone Wars , so clone production evidently stopped at the end of the Clone Wars and within 5 years the clones were becoming too old to be useful as soldiers.
Hence, they were replaced by non-clone volunteers.
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The answer is both yes and no.
By the time of the Galactic Empire there were still Clones as storm troopers, The age acceleration made them age twice the rate as a normal human.
Most of the clones died off.
The Emperor thought that clones of one genetic template were too easy to manipulate (hence why they in scripted Order 66 in the genes of the original clones.)
They enlisted many non clone Humans in the Imperial Army.
They also made different clones from hundreds of genetic templates.
Many years later they would enlist non human troops but in very few numbers.
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This is strictly from the EU, but in Timothy Zahn's Heir to the Empire series there was a mention that the clones were unstable; for example Joruus C'Boath was a clone of a dark Jedi that was protecting the Emperor's museum compound that housed the Spaarti cloning machines, and he was utterly mad.
After the Clone Wars the Empire moved to regular recruits to supplant and replace the clone army.
By the events of the original trilogy most of the Stormtrooper army consisted of regular human recruits.
Now in the Heir to the Empire series
Grand Admiral Thrawn discovers that using force resistant slugs, called Ysalimari, kept the clones from going insane when grown quickly in the Spaarti tubes.
So during Thrawn's reign there were once again clone troopers in the stormtrooper ranks.
But the destruction of the cloning facilities at the end of the Heir to the Empire trilogy meant the production of clones ceased.
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During the prequel trilogy, we learn that the predecessor to the Imperial stormtroopers are all clones. Are the stormtroopers of the original trilogy still clones? If not, where do they come from?
Are Imperial stormtroopers all clones?
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They were initially all clones (of Jango Fett), but other clones and enlisted humans were later added, especially after the Clone Wars. It is said that the Emperor later favored humans.  During the Clone Wars all of the storm troopers were clones, however after the events of the clone wars the Emperor decided to stop the production of clones and allow nowmal humans to enter the storm troopers meaning that there was a mix of clones and humans in the storm troopers.
During the Clone Wars all of the storm troopers were clones, however after the events of the clone wars the Emperor decided to stop the production of clones and allow nowmal humans to enter the storm troopers meaning that there was a mix of clones and humans in the storm troopers.