Barcelona is a good place to introduce major upgrades for two main reasons - traditionally the start of the European season is the first place where it is easy to send significant new parts, and it is also the track the teams know best of all thanks to winter testing.
Red Bull is likely to have the advantage here again because efficient aerodynamics are more important than on other circuits and this is an area at which the team excels. They have said very little about their developments for this race but are likely to have brought in at the very least some more incremental changes to their car.
Their rivals, meanwhile, are throwing everything they can at their cars in an effort to catch up.
McLaren will introduce almost a dozen upgrades after a testing rig failure meant they did not have the confidence to run some of these planned developments in Turkey. The upgrade will include a new front wing, blown diffuser changes and some internal developments.
Ferrari, who overtook McLaren to score their first podium of the season with Alonso in Turkey, will bring in their first aero modifications since they sorted a problem with the calibration of their wind tunnel, and that should see a good step from new front and rear wings.
Mercedes have taken an unusual approach of focusing on what they have rather than bringing in quick upgrades so far. Having made a good step with the set-up of their package in Turkey, however, it is now time to bring in an aerodynamic and suspension upgrade, but team boss Ross Brawn expects this not to upset the recent progress.
Renault, meanwhile, have six individual changes to parts on their car and expect them to add up to a benefit of between 0.15 and 0.2 seconds per lap.
The suggestions are that McLaren's appears to be the most significant upgrade, but that is because it is effectively two upgrades in one due to the issues in Turkey. In general, the race-by-race pace of the front-runners' development progress was summed up well by Renault technical director James Allison, who explained: "These aren't a huge upgrade but if we keep that up every race then it starts to tell."
In contrast, the midfield runners all seem to be focused on step upgrades - and significant ones are planned for this weekend from Sauber and Force India, although the latter may hold on until Monaco to introduce the full package.
Sauber will change their front wing, bodywork and brake ducts and add new devices around the floor leading edge and under the chassis as well as testing the new exhaust system. Force India, meanwhile, ran a new three-element front wing in Turkey along with a sharper nose but plans to add sculpted front floor developments and some major blown diffuser modifications this weekend or next to maximise the improvements in overall aerodynamic flow over the car.
Williams ran a straight line test to check new rear wings and a new exhaust-blown diffuser after Turkey, but Rubens Barrichello has warned the team that they are "rushing things" and that the developments are often not turning out to be as successful as expected, so their main interest will be on how their close rivals' plans work out.
Lotus, meanwhile, has put in a major bid to join this group this weekend with a significant upgrade of its own. The most promising of last year's newcomers, it has progressed forwards already but now expects a big step with a new diffuser and rear end, Red Bull style, supposedly worth up to a second in lap time if it all works out.
It should certainly take them away from Virgin, who had a straight line aero test as they begin to look over their shoulders at HRT, whose upgrade in Turkey was a success and who are planning to bring another step in for their home race this weekend.
But bringing in new parts and making them a success is not easy.
With no testing away from the race weekends, teams have just two sessions on Friday to quickly work out how to maximise the package to gain significantly - and sometimes this is not enough. Several top teams have put development parts on hold at certain races this year due to a lack of understanding.
One advantage with Barcelona is that the teams know it well and, as Lotus driver Heikki Kovalainen explained, it is "one of the tracks where we all know every single centimetre of the whole lap".
All teams aside from HRT, who failed to test before the opening race, have had some experience with their new cars on this track - but already the machines are very different to those that last hit the Barcelona track in March.
Most of the top teams expect to have shaved up to a full second off their lap times in the two months since the last test, but while the configurations now are different and there will also be much hotter temperatures this weekend, the data will help to some extent.
McLaren will gain the least advantage from this testing data because they were still trying to get to grips with their blown diffuser solutions during that test and their car was a totally different machine to the one they are racing with now.
Plans could also be hampered by the introduction of a new Pirelli tyre this weekend. This hard tyre reduces degradation and improves tyre wear so there will now be a bigger gap between the soft and the hard tyre. With a longer pit lane than other circuits, the calculations on how often to stop will be crucial - so teams will need as much time as possible on Friday to modify their race strategies.
Teams with major upgrades may gain from developments but with less time to achieve a race set-up, only careful compromise will achieve success.