Order 66 was one of the most pivotal moments in the Star Wars saga, and many people knew about it before it happened. Order 66 compelled the Clone Troopers to turn on the Jedi, initiating the Great Jedi Purge and clearing the way for Palpatine’s fascist coup. In the current canon, the protocol is top secret, as it was essential to Palpatine’s schemes for overriding the Galactic Republic. Only an essential few knew about the order, and even fewer discovered it before its execution.
Among Palpatine’s essential minions who were privy to Order 66 were his Sith apprentices. Naturally, they’d need to be aware of the order since they’d participate in the Jedi Purge itself. As the designers of the Republic Clone Troopers, the Kaminoans needed to know as well, since they created the control chip implants that overrode the clones’ free will in the first place. Palpatine also allowed those in his innermost circle within the Republic to know, for either personal reasons or as part of his plot to transition the Republic into an authoritarian regime.
Although their control chips removed their free will and turned them into tools of Palpatine, the clones themselves were fundamentally good men with a curious, creative nature. Some clones accidentally discovered their chips and, thus, the nature of Order 66, but Palpatine was able to cover his tracks, silencing those who discovered the protocol, one way or another.
Palpatine
As the mastermind behind the Great Jedi Purge, the formation of the Galactic Empire, and Order 66 itself, it’s no surprise that Palpatine knew about the protocol. Order 66 is one of the many moments that define Palpatine as a master planner and the ultimate villain of the Star Wars saga. Palpatine was able to implement a master strategy to spread the Jedi across the galaxy in a phony war against a corporate oligarchy that was ultimately his puppet regime. The clones themselves genuinely gained the trust of the Jedi, making them the perfect assassins.
Palpatine was also personally involved in silencing Fives, the first clone to learn about the control chips. In one of his most cruel moments, he personally confirmed to Fives that he is the Sith Lord who’s been controlling the war from the start and that the chips would brainwash the clones into killing their Jedi Generals. Palpatine knew that nobody would believe Fives and that Fives’ subsequent attack on him would get him framed as a rogue clone and eventually killed.
Dooku
While Palpatine was the ultimate mastermind behind Order 66, it was in large part set up by Count Dooku. Dooku dutifully carried out the process of selecting a worthy clone template and working with the Kaminoans to implant a control chip in every clone embryo that would compel them to kill their Jedi leaders. Palpatine may have been the brain behind Order 66, but Dooku was the face of the plot.
Like Palpatine, Dooku was also involved in silencing anyone who discovered the chips (and, thus, the protocol) before Order 66 could be executed. In addition to working with the Kaminoans to suppress Fives’ warnings, Dooku also had another clone, Kix, kidnapped and placed in suspended animation to prevent his discoveries from reaching the Jedi. Although Dooku’s work was invaluable to Palpatine’s machinations, he ultimately never lived to see Order 66 successfully executed.
Lama Su
As both the prime minister of Kamino and the Kaminoan in charge of the clones’ creation, it was a necessity that Lama Su was aware of the control chips, and thus Order 66. Despite this, Lama Su was not fully aware of the nature of Order 66 or the chips. According to Darth Tyranus (Count Dooku), the chips were a contingency measure against clone aggression that may stem from their host, Jango Fett. The chip and the order it compelled the clones to follow was also said to be a check on potential rogue Jedi, and thus needed to be kept secret from the Jedi Order.
Because of this, Lama Su was part of a conspiracy to terminate the Clone Trooper Tup (whose chip was malfunctioning and thus was in danger of revealing the truth to the Jedi), and later silence Fives, who did discover the truth.
Nala Se
Similar to Lama Su, Nala Se needed to know about the control chips and Order 66 since she was one of the head scientists responsible for the creation of the Clone Troopers. Nala Se was also only partially aware of Order 66’s true nature. Cold and callous about the clones’ humanity, Nala Se felt no remorse in the planned termination of Tup (which she nearly carried out herself), as she saw the Clone Troopers not as living beings, but products created for Palpatine’s war. Nala Se also was instrumental in the assassination of Fives, drugging him on the voyage to Coruscant so that he’d appear insane and unstable upon learning the true nature of Order 66 and Chancellor Palpatine.
Mas Amedda
Sometime after Senator Palpatine became the Supreme Chancellor of the Republic, Mas Amedda, a corrupt politician with authoritarian leanings, became part of his inner circle. It wasn’t long before Palpatine shared the secret of his Sith Lordship with Amedda, and most likely Order 66 as well. Given how instrumental Order 66 was to Palpatine’s plans, his close minions within the Republic must have been kept in the loop. Amedda was obviously extremely loyal to the secret Sith Lord, and he assisted him in both the assassination of Fives and spreading the false narrative that Fives and Tup’s behavior was due to an exotic parasite.
Sly Moore
Sly Moore was a Force-sensitive Umbaran and, like Mas Amedda, a member of Palpatine’s innermost circle who was aware that he was also the Sith Lord Darth Sidious. In the current canon, Moore is never directly involved in any of Palpatine’s schemes, but it can be assumed that she was at least aware of Order 66, given her knowledge of Palpatine’s Sith identity.
In the Expanded Universe (aka Legends), Moore was far more than simply a confidant of Palpatine’s; she was trained to use the dark side by Palpatine, assisted him in maintaining his dual identity, and was even in a romantic relationship with him (and was likely the mother of Palpatine’s son, Triclops). There would be few to no secrets between Palpatine and Moore, so her knowledge of Order 66 is a given.
Fives
The first of a small number of individuals to learn about Order 66, Fives discovered the existence of the clone control chips after his friend, the Clone Trooper Tup, began attacking Jedi when his chip malfunctioned. Investigating the chips himself, Fives eventually became entangled in a conspiracy to silence him before his research could interfere with Order 66. Fives was the only clone to learn the truth about Palpatine’s Sith identity and the true purpose of the control chips. Though Fives was framed, murdered, and had his investigation covered up by Palpatine and his agents, his warnings did get through to at least two other clones.
Rex
While the Clone Captain Rex didn’t learn the full scope and intention of Order 66 before it was executed, he did heed Fives’ warnings about the control chips after the latter’s death. Rex filed an official complaint about the chips, and likely only suspected their purpose to be similar to what the Kaminoans were told: The chips were implanted as a contingency against clone aggression and potential rogue Jedi.
Ultimately, Rex’s chip was removed by Ahsoka Tano before he could kill her, and Rex went on to help remove the chips from two other clones: Wolffe and Gregor. The three would later join the Rebellion against the Empire and fight for their own ideals.
Kix
Similar to Rex, the Clone Medic Kix took Fives’ warnings seriously and was wary of the control chip implants. Unlike Rex, Kix went a step further than filing an official complaint about the chips, opting to investigate them himself, as Fives did. Kix eventually discovered the truth about the chips and Order 66 and hoped to warn the Jedi of their potential doom, but unfortunately, his discovery reached the notice of Count Dooku.
Hoping to avoid a similar incident to Fives’ discovery, Dooku had Kix kidnapped and placed in stasis instead of assassinated. Before Kix could be brought to Dooku’s homeworld, the vessel he was on was shot down by Republic forces, and Kix wasn’t revived until the war between the Resistance and First Order, decades after Order 66 was successfully executed.
Maul
Maul was not only Palpatine’s Sith apprentice but was essentially his adopted son, having been raised by the Sith Lord since infancy. As a result of both his upbringing and prophetic visions, after he ceased to be Palpatine’s apprentice, Maul was aware of his former master’s plan for galactic domination, as well as his hopes to recruit Anakin Skywalker to the Sith Order. Despite this, Maul was unaware of the specifics of Order 66.
As a bitter former Sith bent on revenge against his master, Maul was intent on disrupting Palpatine’s plans at the end of the Clone Wars, at one point trying to convince Ahsoka Tano to join him in this lofty endeavor. Tano didn’t believe Maul’s warning and captured him with the help of her Clone Troopers. Maul learned the truth of Order 66 from Ahsoka after the protocol was activated. With one of the few missing details to Palpatine’s plot now revealed to Maul, he found the sinister cleverness of the order amusing.
Yoda
In the recent anthology book, The Clone Wars: Stories of Light and Dark, it’s revealed that Yoda received visions of Order 66 early in the war. While Yoda cared for the clones’ humanity, despite their apparent purpose, he felt a sinister undercurrent to their programmed obedience, a feeling that, had he acted on it, could have prevented Order 66 and thus the Jedi Purge. When Yoda learned about the conspiracies behind the late Jedi Master Sifo-Dyas and the clone Fives, it becomes even more baffling that he couldn’t understand the impending catastrophe of the protocol. Ultimately, it would seem that his trust in the clones and refusal to pursue a vision prevented him from interfering with Order 66 himself.
The Entire Grand Army of the Republic (Legends)
In the Star Wars Legends continuity, Order 66 was one of 150 contingency orders that were implemented in case of emergencies. As shown in Republic Commando: True Colors, the orders were common knowledge to everyone in the Republic Army, from the rank and file Clone Troopers to the Jedi Generals themselves. It can be assumed that the Jedi never thought that the order would be activated, given their democratic ideology and loyalty to the Republic. If only the Jedi had inspected the order’s specific wording, they’d see that it mandates their removal by lethal force and that a new command structure would be later established. These are all red flags of impending authoritarian rule, and the Jedi trusted their system too much to take notice and were ultimately too late to prevent their destruction as a result.