The allegation that a researcher in Parliament may have been spying for China has thrown a spotlight on Chinese intelligence activity in the UK - and whether the UK has been slow to respond.
Spying used to be fairly straightforward. An intelligence service recruited an agent to steal or photograph some documents stamped 'Top Secret' from a safe and pass them on at a clandestine meeting.
That was the world in which the UK's 1911 Official Secrets Act was passed.
But much has changed since then.
Now, there is cyber-espionage - the stealing of secrets remotely over computer networks.
And the type of information being sought has also changed.
In 1911, the fear was Germany getting hold of plans for the latest Royal Navy ship. Now, it might be the scientific research on bio-engineering at a university which has never been formally classified as a government secret.
And intelligence services do more than just steal secrets. There is covert influence and interference. This might involve finding people who know the inside track on a policy debate - say about sanctions - and who might be able to shape it in a particular way. This can be hard to distinguish sometimes from diplomacy and lobbying (hiding who you really are is often the best sign).
And in this new world, China poses a particular challenge. It has vast and well-resourced intelligence services - probably the largest by number in the world.
It also engages a wider range of people than just spies to carry out its work - including companies and individuals - part of what is described as a 'whole of state' approach.
The Chinese services often sweep up as much information as they can - some of it not what is typically considered secret and which would not have been of interest to Britain's MI6.
This can all make it harder to spot.
This summer's report from the Parliamentary Intelligence and Security Committee (ISC) outlined some of the ways China operates - including when it comes to politics.
"It appears that China has a high level of intent to interfere with the UK government, targeting officials and bodies at a range of levels to influence UK political thinking and decision-making relevant to China," the committee said.
This included encouraging people whose views align with the Chinese Communist Party to seek political office. The ISC report also said MI5 investigations of Chinese intelligence officers found one of those gaining access to at least one UK parliamentarian.
One of the problems is that the UK seems to have been slow to wake up to the challenge.
In the early part of the last decade, the government was pursuing closer links with China but even as late as 2019, UK security agencies were saying that preventing interference - as opposed to espionage - was not primarily their job but instead one for other government departments, for instance education when it came to activity in universities.
This led to an 'intelligence gap' where Britain's spies were not seeking to identify the activity since it was not their job to deal with it. Meanwhile other departments lacked the expertise or ability to do this.
"The nature of China's engagement, influence and interference activity in the UK is difficult to detect, but even more concerning is the fact that the government may not previously have been looking for it," the ISC said.
The security services have also long complained that they did not have the tools to confront the new reality of what foreign intelligence services get up to. In particular, they argued the Official Secrets Act was not fit for purpose. Under the UK's laws, even being an undercover intelligence officer for China or Russia was not in itself illegal.
The government did finally pass new a new National Security Act this summer.
This will allow people to prosecuted for being or helping an undeclared spy. But this power is not yet in effect - officials say it is due by the end of the year.
Another part of the act is due to set up a Foreign Influence Registration Scheme in which people will have to register if they are working on behalf of a foreign state, with the risk of prosecution if they do not. But this also is not ready and a further consultation means it will not be up and running until at least next year. And it is not yet clear if China will be placed on the 'enhanced' tier which would mean extra checks.
Then there is the political context - does the government want to keep relations with China on a stable footing for the sake of economic investment and trade and if so what does that mean for the appetite to take action?
And so while the challenge from China may now have been identified, dealing with it may still not be straightforward.