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...
21. Shrewsbury
The Shrews were delivered a blow when talisman Josh Laurent turned down a new contract to join Championship side Reading. Versatile defender Omar Beckles also has left the club and is reportedly on trial with Wigan. Photo: Jason Brown
22. Sunderland
Commanding goalkeeper Jon McLaughlin decided to leave the Stadium of Light to join Ranngers. Sunderland also released players like Ethan Robson, Kyle Lafferty, Tommy Smith and Joel Lynch. Photo: Frank Reid
23. Swindon
Eoin Doyle bagged 25 goals in 29 games as the Robins clinched the League Two title, but opted to join Bolton this season. The Robins lost ex-Blues pair Danny Rose and Lloyd Isgrove, while Jerry Yates - who scored 13 goals last term - moved to Blackpool. Photo: Dan Mullan
24. Wigan
The cash-strapped Latics have been forced to sell the majority of their prized assets after entering administration. Former Pompey winger Jamal Lowe was sold to Swansea, along with the likes of Antonee Robinson (Fulham), Cedric Kipre (West Brom), Kieffer Moore (Cardiff) and Joe Gelhart (Leeds) - all for cut-price fees. More departures are expected. Photo: Charlotte Tattersall