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...
5. Burton
The Brewers will be gutted midfield dynamo Scott Fraser turned down a new deal. Burton have also lost centre-back Richard Nartey after his season-long loan from Chelsea and Jamie Murphy, who bagged seven goals in just nine times after arriving from Rangers on loan in January. Photo: .
6. Charlton
Much was made about striker Lyle Taylor's decision not to feature for Charlton after football returned following lockdown. He's now joined Nottingham Forest. Charlton have also seen ex-Pompey target Tom Lockyer depart on a free to Luton. Centre-back Naby Sarr has left The Valley, too, while Josh Cullen has returned to West Ham after his loan expired. Photo: .
7. Crewe
Crewe have kept a lot of their League Two promotion-winning squad together, with the likes of Perry Ng, Harry Pickering and Charlie Kirk still at the club. Michael Nottingham made a good impact when he arrived on loan from Blackpool in January but his loan has since expired. He started the Seasiders' Carabao Cup loss at Soke last week. Chuma Anene netted nine times during his loan from FC Midtjylland. Photo: Lewis Storey
8. Doncaster
Winger Kieran Sadlier snubbed a new deal to join Championship outfit Rotherham. The loans of Ben Sheaf (Arsenal), Seny Dieng (QPR), Niall Ennis (Wolves) and Jacob Ramsey (Aston Villa) have also expired. Photo: .