4K, Ultra HD, 4K Ultra HD – call it what you want, the 'four times HD' TV technology arrived in force in 2014, with second-generation 4K TVs, broadcast trials and even some 4K content.
Now in 2015, we have more 4K TVs on the market than ever, content from Amazon Instant Video, Netflix and more, and Ultra HD Blu-ray discs and players have been confirmed for release by the end of 2015.
We've seen many Ultra HD TVs pass through our test rooms from all the major brands, and some that are best avoided, and we can resoundingly say we're impressed. We've even seen a 4K OLED TV from LG that produces what is (arguably) the ultimate TV performance.
So, what's it all about? On this page we'll run through the basics of ultra high-definition video, and get you up to date with the latest 4K news and available content.
We'll be keeping this page updated as your one-stop for everything Ultra HD, so if you're in the market to buy a 4K TV, check back here regularly...
MORE: Best 4K TVs to buy 2015
4K TV resolution
Officially, 4K resolution is 4096 x 2160 pixels. However, in order to shoehorn this higher resolution video on to a normal 16:9 picture format, it has been altered to 3840 x 2160 – still four times the total number of pixels on a Full HD 1080p screen (1920 x 1080).
In order to take full advantage of 4K Ultra HD you will, of course, need a compatible TV, a source and content that packs those all-important extra pixels.
And that's where it gets slightly messy. While we've been reviewing 4K TVs since 2012, it's only really this year that we've had any content to get our teeth into. So if you've resisted 4k until now, you've made the right decision - and your patience is about to pay off.
Netflix 4K streaming arrived last year, with House of Cards: Season 2 making history as the first 4K streaming content from the service, and Breaking Bad in 4K following soon after. More content, mainly original content, is being added all the time, so it's worth keeping an eye out for the latest releases.
However, you will need a 4K TV with the HEVC codec that Netflix is using in order to play the content. Sony and Netflix struck a deal to ensure Sony TVs get the best out of Netflix, but we've since seen compatible HEVC TVs from LG, Samsung and Panasonic.
Amazon has a large collection of 4K content in excess of 90 hours worth, with more being added all the time. The retail giant's Original content comprises mainly TV shows, and these are available to stream for free through the Prime Instant Video service. Other content, a mixture of films and TV shows, is available to stream at a cost.
YouTube also has a selection of 4K videos but unless you're watching on a 4K TV, you'll need to make sure you have a compatible 4K monitor. And for those who have the required hardware, there's even an 8K video. YouTube uses the VP9 codec developed by Google, and it's been running in the Chrome browswer since 2013. As with HEVC, you'll need to make sure your TV or monitor support VP9 before trying to stream.
Other options? If you're in the US, there's the Sony 4K Ultra HD Media Player, which went on sale in July 2013 for $700, and has since been followed by the FMP-X5 media player, which can bring 4K streaming content to 2012 and 2013 TVs
The FMP-X1 server comes pre-loaded with ten 4K films, has a 2TB hard drive, HDMI and USB connections and connects to your network via an Ethernet connection - and it sounds like you'll need that wired connection for accessing Sony's 4K download service.
Officially called Sony Video Unlimited 4K, the download service is live in the US and promises to be the world's first network service to give access to a constantly updated library of 4K films and TV shows.
Philips has also announced its UHD 880 4K media player, a little black box for sending 4K content to Philips TV.
NVIDIA has released the Shield Android games console that's also capable of streaming 4K as it supports all the required codecs. It's now available to buy in the US, but a UK release date is "coming soon."
Read our complete 4K content guide to find out what you can watch and where.
The last time we updated this section, the 2014 World Cup had taken place, and the BBC had conducted 4K broadcast trials with Sony and FIFA. We also thought that we were still some way off live 4K broadcasts.
But BT has since launched the BT Sport Ultra HD channel and accompanying Ultra HD box in the UK. While the channel isn't broadcast over airwaves, instead relying on an Internet connection, it's definitely a step in the right direction.
Sky has also pledged its commitment to 4K broadcasts and is planning to release a new SkyQ box, although it still isn't clear just how many channels it plans to show in 4K. Sport will likely be a priority, and in August 2013, it carried out its first ever live 4K trial broadcast using the English Premier League match between West Ham and Stoke City.The BBC has said it plans to broadcast 4K as standard by 2016, with the Rio de Janeiro Olympics seen as the key event in the calendar. A recent BBC survey claimed 23% of viewers could benefit from 4K TV, based in part on the average TV screen size and viewing distance.
The BBC and Sky are also part of the UHD-Forum, which has been formed to promote 4K TV. Led by the Digital TV Group, the forum aims to avoid the confusion that emerged when HD TV and HD-Ready TVs first came on sale.
There's an outside chance that Sony will produce a firmware update to enable 4K video on the PlayStation 4. That would allow far more people to access Ultra HD video, but don't hold your breath.
As ever, it seems that Japan is way ahead of us...
More after the break
Ultra HD Blu-ray
After plenty of rumours, Ultra HD Blu-ray is now official.
The official spec, name and logo have been confirmed, and we can look forward to players and discs being unveiled in September at IFA 2015 in Berlin. The Blu-ray Disc Association has announced that manufacturers can apply for an Ultra HD licence from August 24th 2015 and expects compatible hardware to be in stores for the holiday season.
Panasonic unveiled prototype Ultra HD Blu-ray player at CES 2015, but since then news of other brands releasing hardware has been pretty thin on the ground. With the new licensing news, expect that to change over the next few months.
We spoke exclusively to Ron Martin, Blu-ray Disc Association board member and Panasonic Hollywood Labs VP, who revealed some more details on prices and release dates.
He also told us more about the importance of the digital bridge, HDR and why it will deliver better than 4K Netflix pictures.
4K TV prices
This year we've finally seen budget 4K TVs, from ultra-cheap Chinese manufacturers to affordable sets from the big-name brands and own-brand Ultra HD TVs in UK retailers - a long way from the likes of the Sony KD-65X9005A, which came in at £6000 back in 2013.John Lewis has announced affordable Ultra HD TVs, with three sets starting at a 40in TV for £700, while the general 2015 TV ranges are now far more heavily weighted in the direction of 4K sets, bringing down the average cost.
If you want to break the bank you can, thanks to 4K OLED TVs such as the LG 55EG960V, yours for £3800. You can also pick-up top-notch TVs for around half the price, including the award-winning Samsung UE55HU7500.
To make the most of Ultra HD, we wouldn't recommend using anything too small, but a much wider choice in both screen size and price can only be a good thing for the technology.
See all our 4K TV reviews
Apple 4K TV
The Apple iTV has been talked about for longer than many of us can even remember (well, a few years at least) and after rumours of a TV with AirPlay and Siri, or even an OLED Apple TV, the situation is still no clearer.
On the one hand, you have billionaire businessman and influential Apple investor, Carl Icahn, who seems convinced Apple will soon enter the Ultra HD TV market (and car market, for what it's worth). But then maybe he would say that.
The flipside is a recent source, quoted by the Wall Street Journal, that claimed Apple did indeed have a team dedicated to working on an Apple television, but that team has now been disbanded.
We have to say, our money would be on the latter.
4K TV verdict
If you previously doubted whether 4K would ever make the breakthrough into the mainstream - and we wouldn't have blamed you - it does now look like the resolution is here to stay. As such, it has become a real consideration for when you buy your next TV.
4K screens are cheaper than ever, available in more sizes than ever, and, crucially, there's now some Ultra HD content to watch.
We're now happy to recommend a number of Ultra HD 4K TVs, all of which deliver the necessary picture performance for SD, HD and 4K content without breaking the bank.
Streaming 4K video has arrived (albeit with limited content), 4K broadcast plans are well and truly underway, and Ultra HD Blu-ray is now a reality.
Sold? In that case, you might want to check our round-up of the best 4K TVs you can buy...