Ronaldo was born in Bento Ribeiro, a poor neighborhood in Rio de Janeiro. Like many of his friends, he began to play football in the streets of his neighborhood. Ronaldo's actual date of birth is September 18, however, his father did not register his birth certificate until September 22,^{[citation needed]} so some discrepancy has existed as to his actual birth date. His football abilities were first recognised when he was 14. He was recommended to the Brazil youth team by World Cup winner Jairzinho, who also arranged for his own former club, Cruzeiro Esporte Clube, to sign him when he was old enough for a professional contract.^{[citation needed]}

Ronaldo scored 12 goals in 13 games in the Brazilian National Championship, and in the Minas Gerais State Championship he scored all three goals in Cruzeiro's 3-1 victory against arch-rival Atlético Mineiro. After being scouted by Piet de Visser, he was soon transferred for US$6 million to PSV Eindhoven, where he scored 42 goals in 46 league games and reached a total of 55 goals in 57 official appearances. Later he attracted the attention of FC Barcelona. He played for Barça in the 1996-97 season, scoring 47 in 49 (including appearances in the Copa del Rey and European Cup Winners Cup) on the way to leading the Catalan side to UEFA Cup Winners' Cup triumph (where he capped the season with the winning goal in the cup final itself). Inter Milan swooped to sign him the following year, and Ronaldo duly helped them repeat his former side's cup-winning run, this time in the UEFA Cup. As of 2006, Ronaldo is the last player to score more than 30 goals in 1 season in the Spanish La Liga.

On 21 November 1999, during a Serie A match against Lecce, Ronaldo felt his knee buckle and was forced to limp off the pitch. Medical exams after the match confirmed that the striker had ruptured a tendon in his right knee and would require surgery. ^[1]During his first comeback on April 12, 2000, he played only seven minutes during the first leg of the Coppa Italia final against Lazio before injuring his knee for a second time.^[2]

After two operations and 20 months of rehabilitation, Ronaldo came back for the 2002 World Cup, helping Brazil win their fifth World Cup title. Later in 2002 he won the World Player of the Year award for the third time, and transferred from Inter to Real Madrid for approximately €39 million. Even though he was sidelined through injury until October 2002, fans continued to chant his name in the stands. Ronaldo scored twice in his debut for Real Madrid. That same reception was observed on the night of the final game of the season against Athletic Bilbao, where Ronaldo scored again to seal his first season with 23 league goals and the La Liga Championship title for 2003, which Ronaldo had previously failed to win while with Barcelona.

On 18 January 2007 it was reported that Ronaldo has agreed terms with A.C. Milan for a transfer of about €7.5 million.^[3] Ronaldo was forced to pay for the remaining period on his contract which tied him to Real Madrid, only because the latter did not accept to release him, while A.C. Milan were not ready to pay such a sum. On Thursday, 25 January Ronaldo flew from Madrid to Milan to watch A.C. Milan in a cup tie against AS Roma. Statements on the club's website said that Ronaldo was in Milan for a medical, and that a meeting had been arranged for Monday with Real Madrid officials to discuss and finalize his transfer to Milan. On January 26 Ronaldo successfully completed his medical tests at the Milanello training complex under the supervision of club doctors, and the transfer completed on 30 January^[4] and got the jersey number 99. He made his debut as a substitute for Milan on 11 February 2007, during the 2-1 victory over Livorno. The next game at Siena on 17 February 2007, Ronaldo scored twice and assisted on a third goal in his first start for Milan as they won an exciting game 4-3. In his first season at Milan, Ronaldo scored 7 goals in 14 appearances. ^[5]

After his move to A.C. Milan, Ronaldo joined the list of the few players to have played for both Inter Milan and A.C. Milan in the Milan derby and is the only player to have scored for both sides in the derby game (for Inter in the 98/99 season and for Milan in the 06/07 season). Ronaldo is also one of the few players to have started for Real Madrid and FC Barcelona, which also boasts a heated rivalry. However, Ronaldo has never transferred directly between the teams in the derby. Ronaldo has only played 300-plus minutes for Milan this season and before a Champions League game against Benfica he pulled a calf muscle while warming up. This has put negotiations of signing a new contract under threat as the Milan coaching and general staff are becoming frustrated with his fitness, which his current one run until June 2008. Ronaldo's only goals in the 2007/2008 season, besides his goal against Lecce in pre-season, came in a 5-2 victory against Napoli at the San Siro, where he scored an emotional brace. It was also the first time Milan's much hyped attacking trio of Kaka, Pato and Ronaldo, known as Ka-Pa-Ro played together. Despite his tremendous success over the past decade, Ronaldo never won the UEFA Champions League in his club career. During the 2006-07 season, though Milan won the 2006-07 title, Ronaldo was cup-tied with Madrid and ineligible to take part. The closest that he has even been was in 2003 when he helped Real Madrid to the semi-finals, in which they lost to Juventus F.C..

On February 13, 2008, Ronaldo suffered a severe season-ending knee injury while jumping for a cross in Milan's 1-1 draw with A.S. Livorno Calcio, and was stretchered off and taken to a hospital. Milan confirmed after the match that Ronaldo had ruptured the kneecap ligament in his left knee. It marked the third such occurrence of this injury, which he suffered twice to his right knee in 1998 and 2000. ^[6]

Over a month after his season-ending knee injury with AC Milan, the three-time FIFA World Player of the year ruled out the possibility of retirement and told calciomercato.it that he is certain he will play again. ^[7]