This year the deadline falls at 7pm on September 1, with that the cut-off until player trading recommences in the new year.
As ever, it’s been a summer where names have come thick and fast in connection with Pompey.
Some of those reported talks, some have gone on to sign for the Blues with others, well, carrying less substance.
We’ve kept our ears to the ground, so here’s every player linked with Pompey so far - with some insight into the credibility of those reports.
21. Pelle Mattsson
A major target this summer but Pompey could never agree a deal with Silkeborg for their man. Looked like he was off to Norwich, but they instead signed Mirko Topic and Mattsson is still with his Danish club. | Midtjyllands Avis: Photo: Midtjyllands Avis:
22. Griffin Yow
Pompey were tracking the American winger who's with Belgian side Westerlo, but didn't quite fit the bill for John Mousinho earlier in the summer. | Getty Images
23. Mark Kosznovszky
A deal seen to fruition this summer, though not a player John Mousinho was aware of when his name first surfaced. The Hungarian became a live option as thing progressed into July with a £1m deal agreed towards the end of the month. Photo: Simon Roe
24. Abubakr Barry
Gambian with Austria Vienna, a deal for the midfielder has never been close. | Austria Wien