iPhone 4S (fifth generation)
Launched: October 2011
Apple pulled out the "S" tag again for the fifth generation handset, indicating a minor upgrade rather than a major overhaul.
The design of the iPhone 4S was the same, but inside was Apple's new A5 processor, vastly improved graphics capabilities and an 8MP camera with 1080p video recording. iOS was evolving at the same rate as the hardware, of course, and Siri began life as an iPhone 4S exclusive.
The improvements were enough to persuade us to describe it as "the best thing Apple has ever created" in the official TechRadar review.
iPhone 5 (sixth generation)
Launched: September 2012
Six handsets later, Apple finally decided it was time to tweak the iPhone's screen size and aspect ratio.
Coming in at 20% lighter than its predecessor, the 2012 iPhone adopted a 4-inch screen running at 640 x 1136 pixels.
Otherwise, despite the usual speed bump and a stronger antenna, it was very much business as usual in terms of the design and capabilities.
Our biggest gripe in our iPhone 5 review was with the aging iOS, but with iOS 7 arriving on September 18 that issue is very much negated, which will please a number of iPhone 5 users who've been holding onto the handset for nearly a year.
iPhone 5S/5C (seventh generation)
Launched: September 2013
The big step in the seventh stage of the iPhone's evolution was the arrival of the iPhone 5C, a slightly cheaper, plastic-backed model to help battle Android in the busy mobile middle market. The signs were there already — remember Apple kept the iPhone 4 and 4S on sale during the iPhone 5 era.
Apart from the plastic shell and larger battery though the iPhone 5C was a carbon copy of the iPhone 5 in terms of specs - which was retired to make way for the two new handsets.
As for the flagship iPhone 5S, it was a case of under-the-hood improvements again: more power, a better camera, and a fancy fingerprint reader hidden under the home button. The bigger changes arrived with iOS 7, the most radical revamp of the mobile operating system since the App Store arrived back in 2008.
iPhone 6/6 Plus (eighth generation)
Launched: September 2014
After the smaller changes that came with the iPhone 5C and iPhone 5S, Apple was back to its revolutionary best with the following generation as the iPhone 6 and iPhone 6 Plus brought the biggest alterations in design and features since the leap from the iPhone 4S to the iPhone 5.
The most striking change the eighth generation iPhones ushered in was undoubtedly the screen size, with the iPhone 6's display leaping from the iPhone 5's 4-inches to 4.7-inches. Apple also upped the resolution from the 1136 x 640 of previous iPhones to 1334 x 750. This meant that the larger screen still had a high pixel density of 326ppi (pixels per inch), so image quality remains incredibly sharp and detailed.
As the name suggests the iPhone 6 Plus went even larger with a whopping 5.5-inch display, marking Apple's first foray into the phablet market. The iPhone 6 Plus also got a resolution boost to full high definition, 1920 x 1080. With a pixel density of 401ppi not only does the iPhone 6 Plus have the largest screen of any iPhone ever, but it also has the clearest as well.
The iPhone 6 and iPhone 6 Plus also benefit from Apple's use of "dual-domain pixels", which along with the 1300:1 contrast ratio makes the screen on the handsets look absolutely fantastic.
Both the iPhone 6 and the iPhone 6 Plus also ditched the industrial and sharp-edged design that had been with the iPhone for four generations in favour of a more ergonomically-pleasing chassis with a screen that curves into a body with rounded edges and corners.
Both handsets feature metal backs, and as we pointed out in our reviews they take a lot of design language from the iPad Air, resulting in a product that looks and feels genuinely premium.
Another big change is that the boosted screen sizes and the corresponding increase in body dimensions has meant that the power button now resides on the right-hand side of the devices.
Along with the launch of the iPhone 6 and iPhone 6 Plus Apple released iOS 8, which kept much of the flat aesthetics of iOS 7 and ushered in some neat new features.
It wasn't a completely smooth launch, however, and after only a few months Apple has already patched it to iOS 8.1.3 in a bid to squash bugs and fix problems.
The rocky launch of iOS 8 was in stark contrast to the assured releases of the iPhone 6 and iPhone 6 Plus, with Apple proudly announcing that the two iPhones broke the company's records for pre-orders in the first night they were available.
Launching: September 2015
This brings us bang up to date, so what does Apple have in store for its next iPhone? We're still not sure if we'll see an iPhone 6S or an entirely new iPhone 7.
However we have been hearing plenty of rumours and speculation about the next generation iPhone already, for example it will have "the biggest camera jump ever", featuring a two-lens system which could apparently rival DSLRs in quality.
Sony has recently announced a 21MP stacked CMOS sensor which can focus on fast moving objects and includes HDR for 4K video and high-resolution photos. What's interesting is that Sony currently supplies Apple with camera modules, so we might see some of that tech in the next iPhone.
It will almost certainly come with a new processor, most likely called the A9, and the rumours we've heard suggest that Samsung will build the chip that will be 15% smaller, 20% more powerful and 35% more power efficient than the Apple A8 processor found in the iPhone 6 and iPhone 6 Plus.
Could the iPhone 6S or iPhone 7 follow in the footsteps of the Galaxy Note Edge? We don't think it's particularly likely, but there have been rumours that Apple might try something drastic with the screen.
Rumours once again hint at the upcoming iPhone having a rugged sapphire display, while others hint at Apple going for a smaller 4-inch design in a bid to appease people who weren't too keen on the iPhone 6 and iPhone 6 Plus' increased dimensions.
It looks like Apple could also be preparing to launch iOS 9 alongside the iPhone 6S/iPhone 7, with reports claiming a beta program to test iOS 9 will run over the summer – perhaps to make sure there are no repeats of iOS 8's troubled launch.
Make sure you keep an eye on our iPhone 7 release date, news and rumors article to keep up to date with all the latest rumours concerning Apple's next generation iPhone.