The sweeping Investigatory Powers Bill was given the go-ahead to become law on Tuesday despite intense calls for it to be scrapped by privacy campaigners.
The so-called Snoopers' Charter has been brought in under the guise of fighting increasingly tech-savvy terrorists in the digital age.
But critics say it is an Orwellian blank cheque to spy on everyone, with human rights group Liberty branding the new powers "eye-wateringly intrusive".
So, what is actually going to change? Here's everything you need to know:
Internet connection records
This is the biggie that has sparked repeated calls for it to be scrapped.
Web and phone firms will now be required to store data on the sites a device connects to for up to a year, giving cops, spooks and agencies unprecedented access to the information.
This includes calls, texts, browsing habits and social media info, and will come into play before the end of the year.
But the Government has repeatedly stressed it will not cover users' full browsing history or cover the actual content of communication.
It will also cover existing tactics like the hacking of suspects' devices, which is seen as increasingly important as encryption becomes more sophisticated.
The same goes for bulk tactics used by MI5, MI6 and GCHQ, which allows them to collect vast troves of data which the Government says is crucial to counter-terrorism efforts.
Can spooks monitor me for no reason?
Unlikely. The legislation introduces a"double lock" regime for the most intrusive techniques, so that warrants issued by a Secretary of State will now require the approval of a senior judge.
A new commissioner for investigatory powers has also been created, while those misusing them could face criminal charges. But concerns have been raised about just how transparent the whole process will be and how stringently the ethical rules will be applied.
Home Secretary Amber Rudd said: "The internet presents new opportunities for terrorists and we must ensure we have the capabilities to confront this challenge. But it is also right that these powers are subject to strict safeguards and rigorous oversight.
"The Investigatory Powers Act is world-leading legislation that provides unprecedented transparency and substantial privacy protection."
But Bella Sankey, policy director for campaign group Liberty, said: "It's a sad day for our democracy as this Bill - with its eye-wateringly intrusive powers and flimsy safeguards - becomes law.
"(These measures) open every detail of every citizen's online life up to state eyes, drowning the authorities in data and putting innocent people's personal information at massive risk."
Concern has also been raised that the bill would be used by authoritarian regimes to justify their own state surveillance.
And, already, a petition has gained more than 130,000 signatures calling for it to be repealed, meaning it will have to be debated in parliament... again.