125cc, 250 cc and 500 cc World Championships

In 1994, Aprilia by way of Sandroni, used Rossi to improve its RS125R and in turn allowed him to learn how to handle the fast new pace of 125cc racing. At first he found himself on a Sandroni in the 1994 Italian championship and continued to ride it through the 1995 European and Italian championships.
Rossi had some success in the 1996 World Championship season, failing to finish five of the season's races and crashing several times. Despite this, in August he won his first World Championship Grand Prix at Brno in the Czech Republic on an AGV Aprilia RS125R. He finished the season in ninth position and proceeded to dominate the 125cc World Championship in the following 1997 season, winning 11 of the 15 races.
By 1998, the Aprilia RS250 was reaching its pinnacle and had a team of riders in Valentino Rossi, Loris Capirossi and Tetsuya Harada. The death of two of his friends in a car accident also took a toll. He later concluded the 1998 250cc season in second place, only three points behind Capirossi. In 1999, however, he won the title, collecting 5 pole positions and 9 wins.
Rossi was rewarded in 2000 for his 250cc World Championship by being given a ride with Honda in what was then the ultimate class in World Championship motorcycle racing, 500cc. Jeremy Burgess had shown him the NSR500 and was convinced that the pairing of it with Rossi would bring nothing but success. Retired 500cc World Champion Michael Doohan, who also had Jeremy Burgess as chief engineer, worked with Rossi as his personal mentor in his first year at Honda. It would also be the first time Rossi would be racing against Max Biaggi, another Italian to whom he was often compared by the racing press. It would take nine races before Rossi would win on the Honda but, like his previous seasons in 125 and 250, it would bode well for a stronger second season as he finished second to American Kenny Roberts, Jr..
Rossi won his first 500cc World Championship in 2001 (winning 11 races) in the final year of that class. In the following year, 500cc two-strokes were still allowed, but 2002 saw the beginning of the 990cc four-stroke Moto GP class, after which the 500cc machines were essentially obsolete. In 2001 Rossi teamed up with American rider Colin Edwards for the Suzuka 8 Hours endurance race aboard a Honda VTR1000SPW. The pair won the race despite Rossi's lack of experience racing superbikes.
The inaugural year for the MotoGP bikes was 2002, when riders experienced teething problems getting used to the new bikes. Rossi won the first race and went on to win eight of the first nine races of the season, eventually claiming 11 victories in total.
It was more of the same in 2003 for Rossi's rivals when he claimed nine pole positions as well as nine GP wins to claim his third consecutive World Championship. The Australian GP at Phillip Island in 2003 is considered by many observers to be one of Rossi's greatest career moments due to unique circumstances. After being given a 10-second penalty for overtaking during a yellow flag due to a crash by Ducati rider Troy Bayliss, front runner Rossi proceeded to pull away from the rest of the field, eventually finishing more than 15 seconds ahead, more than enough to cancel out the penalty and win the race.