Pompey so far have brought in two players – Sean Raggett and Cameron Pring - ahead of a renewed promotion push.
Meanwhile, the likes of Peterborough, Sunderland, Charlton and Hull have also been improving their sides as they push for the Championship.
But what about the players that clubs have lost? The departures of key men can often prove detrimental to teams' ambitions.
Scroll down and click through the pages to see who each side will miss...
13. Lincoln
Michael Appleton's Imps will be without Premier League midfielders Jake Hesketh and Conor Coventry, who returned to Southampton and West Ham respectively following their loans. Photo: Daniel Chesterton
14. MK Dons
The Dons have lost their two best players this summer. Deadly marksman Rhys Healey has made a surprise switch to French club Toulouse, while midfielder Alex Gilbey joined League One rivals Charlton. Photo: Nigel Keene
15. Northampton
After winning the League Two play-offs, the Cobblers sold captain Charlie Goode for £1.5m to Brentford. Callum Morton, who netted eight times in just 12 games on loan from West Brom, doesn't look like he will be returning, either, after being linked with Lincoln. Photo: Pete Norton
16. Oxford United
The U's' failure to be promoted meant they couldn't keep hold of Rob Dickie, with the captain joining QPR. Meanwhile, Marcus Browne, who scored against Pompey in the play-offs, and Ben Woodburn, have returned to respective parent clubs Middlesbrough and Liverpool. Photo: Michael Steele