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...
17. Peterborough
Peterborough couldn't stop Ivan Toney from moving up the pyramid after plundering 25 goals last season. He's joined Brentford for a reported fee of £10m and will be a big miss. It remains to be seen whether the Posh will get Sammie Szmodics back on loan, after he impressed following his arrival from Bristol City in January. Photo: Simon Davies
18. Plymouth
The departure of Antoni Sarcevic will be a blow for the Pilgrims. He left for Bolton on a free transfer after 132 appearances and 20 goals for Plymouth. Photo: Getty Images
19. Pompey
The Fratton faithful were hugely disappointed Christian Burgess departed for Belgian side Royale Union Saint-Gilloise. Pompey released Oli Hawkins and Brett Pitman, while Steve Seddon is back at Birmingham after his impressive loan during the second half of the season. The Blues were also unable to prise Cameron McGeehan back to the south coast as he opted to join Belgian outift KV Oostende. Photo: -
20. Rochdale
Key-midfielder Callum Camps has joined Fleetwood on a free transfer, while top-goalscorer Ian Henderson has moved to Salford. In addition, MJ Williams opted for Blackpool and veteran striker Aaron Wilbraham has also departed. Photo: Daniel Chesterton