Memory

Several of Death's Head II's memories. (COMIC: Synchronicity II)

Memories were the mental record of a lifeform's past experiences.

Time Lords

By most accounts, when two or more versions of the same Time Lord met, it caused the time streams to be out of sync, with the younger versions being unable to retain the memories of the encounter. (COMIC: The Heralds of Destruction, Vortex Butterflies, Supremacy of the Cybermen, The Lost Dimension) According to the Twelfth Doctor, this was a natural process. (COMIC: Four Doctors) When multiples of the same Time Lord met, they gained new memories in sequence of their incarnations. (PROSE: The Day of the Doctor) On some occasions however, the Doctor did display retention of meeting their future selves. (TV: The Five Doctors, Time Crash)

When Time Lords died, their memories were uploaded to the Matrix, (TV: Hell Bent) ideally so future generations could consult their knowledge. (PROSE: A Brief History of Time Lords) During the Last Great Time War, the Eleventh General uploaded the Doctor's memories of his more notable encounters with the Daleks to a subset of the Matrix so that the Time Lords could study them in the Dalek Combat Training Manual. (PROSE: Dalek Combat Training Manual)

The Fifth and Sixth Doctors both noted that memories from before their second regeneration were vague. (PROSE: Cold Fusion, COMIC: The World Shapers) According to Madame Xing, by the time the Eighth Doctor visited her, his memory had been interfered with on thirty-seven separate occasions, with eight of them still uncorrected. (PROSE: Halflife)

As his earlier selves fell victim to a time scoop, the Fifth Doctor felt himself being "whittled away" and commented on the importance of memory. (TV: The Five Doctors)

A man is the sum of his memories, you know. A Time Lord even more so.Fifth Doctor

As a result of several of their incarnations being present, all of the Doctors prior to the Eleventh forgot their saving of Gallifrey at the end of the conflict. (TV: The Day of the Doctor) Because they could not remember the true outcome of the war, the Ninth Doctor and his successors suppressed their memories of the War Doctor, not acknowledging him as an incarnation of the Doctor, (TV: The Name of the Doctor) before the Eleventh Doctor learnt the truth. (TV: The Day of the Doctor)

After loudly announcing his status as a Time Lord at the Fluren Temporal Bazaar, (COMIC: Weapons of Past Destruction) the Doctor's memories became a valued prize. After luring the Doctor to her, Addison Delamar sought to auction off his memories only for the Doctor to broadcast them to the orbiting fleets. Overwhelmed by the Doctor's grief, the bidders left. (COMIC: The Bidding War)

When the Eleventh Doctor was partially upgraded into the Cyberiad, he wagered his memories in a chess game against Mr Clever. (TV: Nightmare in Silver) When he later lost his memories, due to fiddling with the telepathic circuits, Clara Oswald helped him restore them via the Twelve Hundred Year Diary. He briefly despaired having them again, mourning all that he'd lost over his lifetime, before Clara convinced him of the good he'd done across the universe. (TV: The History of the Doctor)

In order to prevent Clara Oswald from dying, the Twelfth Doctor tried to use a neural block to erase himself from her memories. Upon learning of his plan, Clara reversed the polarity of the device and the Doctor lost his memories of her instead. As a result of the obvious hole this left in his mind, he was able to partially reconstruct his memories of her but could not recall her face. (TV: Hell Bent) The Doctor's memories of Clara were restored near the end of his life when a glass avatar of Bill Potts allowed him to see her once more. (TV: Twice Upon a Time)

Upon the end of their service to the Division, the Timeless Child's memories of their service was wiped. (TV: Ascension of the Cybermen) After the Thirteenth Doctor had rediscovered her past as this being, she recovered the biodata module holding her erased memories but ultimately decided against opening it. (TV: The Vanquishers)

Iris Wildthyme once placed her memories within the Memory Crystal, which was then sought after by many higher powers who wanted to claim her memories for their nefarious needs. Iris also likened her memory to that of an elephant's. (AUDIO: Wildthyme at Large)

As a result of existing alongside Missy, the Saxon Master did not retain his memories of his time on the Mondasian colony ship, subsequently not even being able to fully recall the regeneration into his successor. (TV: The Doctor Falls) When Missy used an elysian field to regenerate, many of her memories of her current incarnation were lost. (AUDIO: The Lumiat)

Others

Two of the Doctor's companions had eidetic memory: Zoe Heriot (TV: The Wheel in Space) and Melanie Bush. (TV: Terror of the Vervoids)

Jamie McCrimmon and Zoe Heriot's memories of their time with the Second Doctor where erased by the Time Lords before they were returned to their original times. (TV: The War Games)

The Tenth Doctor had to erase Donna Noble's memories of their travels together, to save her mind from being overwhelmed by the Time Lord knowledge she gained from the meta-crisis. (TV: Journey's End)

A memory worm could suppress recent memories, causing short-term amnesia. (TV: The Snowmen, Time Heist)

Clara Oswald read the Doctor's name in The History of the Time War. When that timeline was later averted, however, she had no memory of it. (TV: Journey to the Centre of the TARDIS) The telepathic circuits of a dead TARDIS retrieved some of those lost memories. Proximity to the Doctor's timeline caused Clara to experience memories of Oswin Oswald and Clara Oswin Oswald. (TV: The Name of the Doctor) After being splintered, Clara retained jumbled memories of her echoes, vividly remembering some moments of their lives but recalling other moments only as dreams. (PROSE: Into the Nowhere) By the time she was travelling with the Twelfth Doctor, Clara was able to more vividly recall some of her echoes' memories, being able to call on their knowledge and experiences. (GAME: Lost in Time)

Retcon was a drug used by Torchwood to remove memories of the organisation and/or alien encounters from witnesses. (TV: Everything Changes, Adam, They Keep Killing Suzie, Meat, Something Borrowed, COMIC: Shrouded) The drug was also used by other individuals and organisations, notably the mayor of the trap street, Ashildr. (TV: Face the Raven)

River Song used Mnemosine recall-wipe vapour to delete the memory of her encounters with the first nine known incarnations of the Doctor. (GAME: The Eternity Clock)

The Black Archive of the UNIT was provided with a memory filter as a security system. (TV: The Day of the Doctor)

After Bill Potts's encounter with a puddle of sentient oil in the form of her friend Heather, the Twelfth Doctor intended to wipe her memories of the event. Bill realised his intentions and pleaded him not to. Eventually she gave in but before he could go through with it, she told him to imagine how it would feel if someone did it to him. The Doctor, perhaps remembering how Clara Oswald distorted his own memories, reconsidered his actions and allowed Bill to remember. (TV: The Pilot)

The Silents were genetically engineered so that people would lose their memories of them when they turned away. (TV: The Time of the Doctor) Through the usage of an Eye Drive, people could counteract this effect and retain their memories of the Silents. (TV: The Wedding of River Song) The Doctor later exploited this to make information that the Inforarium sold about him memory-proof. (HOMEVID: The Inforarium)

Within the Dalek casing, the cortex vault catalogued all the memories of the Dalek mutant and suppressed those that may cause the Dalek to stop feeling xenophobic hatred. (TV: Into the Dalek) When the Daleks of the Dalek Asylum's intensive care ward cornered the Eleventh Doctor, Oswin Oswald deleted all the memories of the Doctor from the Dalek pathweb. (TV: Asylum of the Daleks) This gap in their memories caused the war machine of the New Dalek Paradigm to grind to a halt as the Parliament of the Daleks debated the identity of their now unknown foe. When they turned Tasha Lem into a Dalek puppet during the Siege of Trenzalore, her own memories of the Doctor were uploaded to the pathweb, allowing the Daleks to regain their memories of their foe. The return of the memories drove the Prime Minister of the Daleks insane. (TV: The Time of the Doctor; PROSE: The Astounding Untold History of the Greatest Enemies of the Universe)

Though Ashildr had been made immortal, her human brain did not have the storage capacity for all her memories, leading her to begin forgetting her past as a result. (TV: The Woman Who Lived)

In 2022, shortly after reading a letter written to her by the Doctor, Abby McPhail called Cleo Proctor, a fellow member of The Blue Box Files to talk about the letter. Shortly after beginning the call, she forgot what she was calling about. Penny Carter's girlfriend always told Penny that she had the "memory of an elephant". (AUDIO: SOS)

The Testimony Foundation travelled across human history and uploaded the memories of humans just before their deaths into glass avatars, a process that the Twelfth Doctor likened to a human-built version of the Time Lord Matrix. (TV: Twice Upon a Time, PROSE: Twice Upon a Time)

In the 54th century, people could upload their memories into the Memgram network via an experience sphere. When Rose Tyler plugged in to the network, Memgram unlocked her repressed memories of an alternate timeline, (COMIC: The Bidding War) with the Doctor taking her to a colony of Silurians to help her mind cope with the memories. (COMIC: The Lost Dimension)

Jack Harkness left the Time Agency after learning that two years of his memory had been erased. (TV: The Empty Child) He later gave up more of his memories to dupe Addison Delamar, who had bated him with the hope of reconstructing his memories, to save his friends from Delamar's greed. (COMIC: The Bidding War)

Consuming the meat of a Xesto allowed one to experience the memories of the beast's victims. (AUDIO: Tick-Tock World)

By the time he had become the First Movellan, Mark Seven was unable to access his memories dating to before his accident at Beltross Station. After being infected by a computer virus, Mark's system rebooted, allowing his true self to say goodbye to the Tenth Doctor before his pre-Movellan memories were deleted forever. (AUDIO: The Triumph of Davros)