We profile a select few who've played for both clubs.
Anton Ferdinand (WHU 2002-2008, SAFC 2008-2011)
Ferdinand signed his first professional contract for the Hammers in 2002 and made his debut a year later, he went on to make 26 games in his debut season for the club. The following season saw Ferdinand cement his place at the heart of the defence as the club secured promotion back to the Premier League. In his six years at Upton Park, Ferdinand made 163 appearances for the club scoring five goals.
He joined Sunderland at the start of the 2008-09 season for £8 million and made a promising start to his career on Wearside, forging a solid partnership with Danny Collins, however some poor results saw Ferdinand in and out of the team. He went on to make 36 appearances in his first season at the Stadium of Light. Following the arrival of Steve Bruce, Ferdinand found first-team opportunities hard to come by. He left the club for QPR in August 2011 having made 95 appearances.
Don Hutchison (SAFC 2000-2001, WHU 1994-1996, 2001-2005)
Scottish international Hutchison joined the Black Cats in July 2000 from Everton for £2.5 million and he quickly became a key figure for the club as they finished seventh in the league. In his one year on Wearside the Scottish international scored 8 goals in 40 league appearances. Including scoring in the 2-1 win against Newcastle at St James Park in November 2000.
Hutchison first joined West Ham in August 1994 when Harry Redknapp brought him to the club from Liverpool for £1.5 million. In his first spell at the club he made 39 appearances scoring 11 goals. He left the club for Sheffield United in January 1996. He rejoined the club in August 2001 for a then club record £5 million. However he suffered an anterior cruciate ligament injury months after joining the club and following his return to fitness he struggled to secure a regular first-team spot. He left the club for Millwall in 2005 having scored 5 goals in 63 appearances.
George McCartney (SAFC 1998-2006, 2008-2012, WHU 2006-2008, 2012-PRESENT)
McCartney first joined the Black Cats as a trainee in 1998 and made his debut in 2000 against Luton Town. He was made team captain during the 2003/04 season and was named fans Player of the Year the following season as helped the club clinch promotion back to the Premier League. McCartney made 157 appearances for the club during his first spell.
McCartney first joined West Ham in August 2006. He made 25 appearances in all competitions for the Hammers during his first season at the club. The following season he was runner up in the club’s Player of the Year award having featured in all of the team’s 38 Premier League games. McCartney's sale back to Sunderland led to West Ham manager Alan Curbishley resigning after he claimed the player was sold against his wishes.
McCartney’s second spell on Wearside was blighted by injury making only 41 appearances and he re-joined West Ham, initially on loan, at the start of 2011/12 season before making the move permanent before the start of the current season.