The game started at 7pm will be shown on ITV and will also be streamed on ITV Hub for those who will not make it home - or to the pub - in time for kick off.
Southgate said: “We are a team who are still improving, we know where we are.
"We are having success because everybody is working so hard, everybody is working hard on the field.
"We are in this position because guys that are in the squad like (Gary) Cahill, (Phil) Jones, (Danny) Welbeck, (Nick) Pope, (Jack) Butland, (Danny) Rose, the older ones, have been exceptional in their mentality, their attitude to support the team, to train every day.
England vs Croatia: How long is extra time? (Image: GETTY)
"Even though they've not had as much game-time as they would have liked, they are as much of a reason why we are where we are.”
The last time England took on Croatia, they beat them 5-1 to bag a place in the 2010 World Cup.
The game was held at Wembley nine years ago and featured players Steven Gerrard, John Terry and Wayne Rooney, as well as David Beckham coming off the bench.
According to Ladbrokes, England are the favourites to win tonight.
England vs Croatia: The game has gone onto extra time (Image: GETTY)
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Their odds are:
England to win (in 90 minutes) - 6/5
Croatia - 11/4
England to reach the final (either in 90 minutes or extra time) - 8/13
England vs Croatia: England scored during the first half (Image: GETTY)