Born in Olomouc, Czech Republic on 15 May 1993, Tomas joined Chelsea full-time in January 2011 having signed a pre-contract agreement with the club the previous summer.
He spent the first half of the season at his former club, Sigma Olomouc, but trained with Chelsea in midwinter prior to making his debut in the reserves, followed by the youth team in February, going on to make 10 reserve team appearances as we tied up the national league title.
A right-sided centre-half who can play right-back, he made his first-team debut for Sigma Olomouc in 2009/10 and is very athletic and quick and good at one-on-one defending. He is also comfortable on the ball.
In August 2011, he signed on loan with Vitesse Arnhem and, after quickly establishing himself in the side, went on to make 37 appearances and score one goal as the Dutch team finished seventh in the league.
He spent the 2012/13 season at the same club, having in July 2012 signed a contract extension with Chelsea committing him to the club until the summer of 2017.
Kalas netted his first goal of the season in Vitesse's 3-0 win at FC Groningen and enjoyed another impressive campaign.
He returned to Chelsea for the 2013/14 season with Jose Mourinho making him part of the first team squad, although a small leg fracture kept the young player out in the early weeks. He made his debut for the club in the 2-0 Capital One Cup victory at Arsenal, appearing as a last-minute substitute.
Having played for Czech Republic Under-17s, Under-18s, Under-19s and Under-21s, he made his full international debut in November 2012, coming on as a sub in a friendly win over neighbours Slovakia.