Derby have signed midfielder Krystian Bielik from Arsenal for an undisclosed fee and brought in Brighton defender Matt Clarke on a season-long loan deal.
Poland Under-21 international Bielik, 21, has agreed a five-year contract with the Championship club.
He spent last season on loan with Charlton, making 34 appearances as the Addicks were promoted from League One.
"He is a young player and it is a permanent deal, which I think is a very good investment for the club," Derby boss Phillip Cocu said.
"I am very happy he has joined the team.
"He will get better and develop, but for his age he is a very good player."
