t was towards the end of the Doctor's first battle against the dreaded silver Cybermen that the Time Lord collapsed, seemingly from exhaustion. 'This old body of mine is wearing a bit thin,' he muttered before stumbling back to the TARDIS where, before the amazed eyes of his two companions, Ben and Polly, his face blurred and changed, the elderly features giving way to those of a far younger man.

The second incarnation of the Doctor came as much a shock to his companions as to the viewing audience. Gone was the elderly and dignified statesman and in came a clownish, unpredictable figure with a sense of the absurd about him. 'What have you done to BBC1's Dr.Who,' cried Mrs Estelle Hawken in the Radio Times. 'Of all the stupid nonsense! Why turn a wonderful series into what looked like Coco the Clown?' However this was perhaps a minority view and before long the BBC's listings magazine was printing letters of praise.

The second Doctor could be impish, fun-loving and devious, always ready to pull out a recorder and tootle a tune, or to act the idiot in order to gain the upper hand against his enemies. Appearances can be deceptive, however, and the Doctor's skill with words and manipulation was still very much in evidence. This Doctor actively encouraged his foes to underestimate him and played this card to his greatest advantage.

He was also a master impersonator, and loved dressing in a number of disguises over the course of his adventures. These included an old washerwoman, a Redcoat, various soldiers, army officials and indeed any persona that would allow him to continue his fight against evil. As he said to his friends during a battle against the Cybermen on the Moon, 'There are some corners of the universe which have bred the most terrible things … They must be fought.'

The Doctor's transformation into his third persona came about as a result of his being captured and put on trial by his own people for interfering in the affairs of others. The Time Lords sentenced him to be exiled to Earth in the twentieth century and for his appearance to change. Thus the character who stumbled from his TARDIS when it materialised on Earth in the twentieth century was very different from that previously known by viewers.

With this third persona, the mischievous charisma of his second incarnation came right to the forefront and the Doctor became an outgoing man of action. He loved to get involved, whether it was chasing wrong-doers by auto-gyro, hovercraft, water-sled or futuristic car, or using his TARDIS to force the Master into a temporal stalemate. He would dash into action with his cloak billowing in the wind and engage his enemies in swordplay and his own special brand of Venusian martial arts rather than verbal sparring.

This incarnation of the Doctor loved gadgets and his sonic screwdriver and souped up yellow roadster called 'Bessie' were never far from his side. The Doctor still retained his emotional and moral values. He hated to see any form of life suffer, and was on the side of the oppressed, even when they were suspicious and hostile towards the hand of friendship that he offered.

It was a fatal dose of blue radiation from the cave of the arachnid Great One on the planet Metebelis 3 that caused the Doctor to change his form once more. The Doctor had to face his fear and in doing so sacrificed his life for the oppressed people of Metebelis. Thanks to the timely assistance of another Time Lord, K'anpo, the Doctor regenerated once more and was soon to break his ties with UNIT and the Earth and once more become a traveller in time and space.

The new Doctor seemed even more alien than before. He had a quirky dress sense and favoured a wide-brimmed felt hat, a comfortable and voluminous coat with virtually bottomless pockets and an unfeasibly long multi-coloured scarf. With the Doctor's exile to Earth being lifted by the Time Lords part-way through his third incarnation, this new Doctor was able to travel once more and viewed the many worlds he visited with an aloof, almost Olympian, detachment. If there was evil to be fought then he would fight it, but he treated the deaths of individuals as necessary to the greater good. He could be cold and impersonal, but the next moment brimming with enthusiasm for the life of a daisy.

He disarmed opponents with wit, a smile, and, as with the second Doctor, an ability to hide his intelligence. It was those he opposed who took him for a fool that were easiest to defeat. Some however, like Davros, creator of the Daleks, never underestimated the Time Lord and yet still found him elusive and difficult to trap and destroy.

After seven years enjoying adventures in his fourth persona, it was the latest in a line of battles against the Master that caused the Doctor to fall from the gantry of a radio telescope. This fall had been predicted, however, and, to the astonishment of his companions, the Doctor merged with a mysterious being known only as 'the Watcher', the result being another incarnation.

The fifth Doctor was much younger in appearance than any of his predecessors, and had a boyish charm and endearing air of innocence. However his looks belied his intelligence which was put to good use in working out just how to defeat foes as diverse as the burrowing Tractators and the devious Black Guardian.

The Doctor liked to rush headlong into action, often breathless with excitement, and his whirlwind enthusiasm affected those around him, particularly the Australian trainee air-stewardess Tegan, Nyssa, the young girl from Traken, and the Alzarian youth Adric. As this Doctor appeared younger than the others, he often took on a more 'elder brother' role with his companions as opposed to the father figure of earlier incarnations.

The Doctor was trusting and open, especially with his companions, and yet still intolerant of evil. His handling of the Trion exile Turlough was especially sensitive, particularly as the boy was tasked by the Black Guardian to destroy the Doctor and his ship.

As might have been expected from the boyish fifth Doctor, he sacrificed his own life so that his companion, Peri, might live. They had both contracted the fatal disease spectrox toxaemia and there was only enough of the antidote for one …

Where the fifth Doctor had been somewhat staid in his outlook, his only affectation being an eternally fresh stick of celery on his lapel, the sixth Doctor was positively ebullient. Perhaps because the regeneration was brought on by a fatal bodily poison, or perhaps because the Doctor was weakened by the illness, he initially suffered severely from post-regeneration trauma. As Peri discovered, he became unstable and violent, and attacked her physically as well as verbally. Moods of fatalistic melancholy gave way to poetic discourse and bewildering fits of bravado and cowardice. As he recovered from the regeneration, so these atypical seizures faded away, leaving a more mellow Doctor.

He could still be abrasive and brash but there was a human side to him also, one which he tended to keep hidden. However on occasion, when pondering on the destruction of Earth perhaps, or tenderly holding a dying friend, his sensitivity would come to the fore.

This Doctor was certainly not one to fade into the background, and his preferred mode of dress was a totally tasteless outfit comprising a gaudy coat of clashing colours, textures and patterns, green spats, patterned waistcoat and black and yellow striped trousers.

This incarnation of the Doctor never really got a chance to shine as no sooner had he started to enjoy his adventures than the Time Lords pulled him out of time to stand trial once more. It could be said that we never really got to know the sixth Doctor.

After a somewhat dynamic and brightly-flaring sixth Doctor, it perhaps seems strange that his next regeneration would be triggered apparently by his falling to the floor of the TARDIS during an attack by the Rani, another renegade Time Lord. He was promptly kidnapped, drugged and mentally confused by the Rani and it wasn't for some time that the Doctor realised who he really was.

Not surprisingly, this treatment resulted in an initial period of uncertainty and instability, but the Doctor's emerging character featured traits from all his predecessors: wisdom, unpredictable clownishness, a love of gadgets, mystery, the ability to be underestimated and a volatile temper.

This Doctor was also seen to have hidden depths and undercurrents. There was a new sense of mystery and intrigue over his origins: he hinted at being more than just a Time Lord, and a witch from the 17th Century seemed to know many 'secrets' about him, all of which remained untold. If the Doctor was a Time Lord, then his role in the history of that race was in the process of being reassessed.

The seventh Doctor could be dark and manipulative, secretive and angst-ridden, keeping his cards very close to his chest and only allowing others to glimpse a fraction of his true motives and aims. He could also be unpredictable and flippant, enjoying teasing others with his apparent lack of concern when, inside, he was plotting and scheming a way to save everyone. He could happily delay the Gods of Ragnarok with an endless presentation of magical tricks and feats of dexterity and could even use some form of psychic power to incapacitate those who threatened to stand in his way.

The eighth incarnation of the Doctor came into being through the most traumatic and violent regeneration that the Doctor had yet experienced. Shot by gangland thugs, rushed to hospital and then anaesthetised and subjected to a heart probe by specialists, the Doctor's alien body was unable to cope and, as far as those attempting to save his life were concerned, he died on the operating table.

Some two hours later his body convulsed and regenerated in the morgue. The new Doctor, staggering from a cold-store dressed only in a sheet was understandably confused. When he began to recover his lost memories, it was to rediscover himself with delight.

This delight was what epitomised the eighth Doctor. He simply loved life. He loved the way a new pair of shoes fitted, the night air, remembering fireworks in the sky … all the simple things that people often take for granted. Of course, given that the Master was again scheming to take these away by destroying the Earth gave the Doctor the impetus he needed to gather some good companions around him and to again take up the fight for the Earth.

The Doctor seemed to have picked up some new traits as well. He was an accomplished pick-pocket, able to steal away security passes or pistols with equal ease. He was also far more emotional than previous incarnations, feeling no embarrassment about kissing his companion in triumph as he remembered who he was.