The Open Skies Treaty allows countries to carry out unarmed surveillance flights over each other's territory.
The flights began in 2002 in an effort to build trust between the treaty's 34 signatories.
However, the US has accused Russia of abusing it and is threatening to withdraw from Open Skies next month.
The House of Lords International Relations and Defence Committee has been told Donald Trump's decision to pull the United States out of a major defence agreement could allow Russia's activities to go unchecked.
What is the Open Skies Treaty?
"The Open Skies Treaty was a treaty that came out of the end of the Cold War as a way to build confidence in military activities, that they were obeying all of the different restrictions that were negotiated at that time," Dr Patricia Lewis from the International Security Programme at Chatham House said.
She added the Treaty focused on what was called "the Atlantic to the Urals space", the area going from North America through Europe up to the Ural Mountains.
What does the Treaty allow signatories to do?
Dr Lewis explained that the idea behind the Open Skies Treaty was that signatories could fly aeroplanes over other signatories' territories.
"Aeroplanes were things that all the states had access to, so it wasn't just those that had satellites in the sky that could see what was going on on the ground," she said.
She added that at the time, aeroplanes were "more important" when it came to assess a situation and understand "what was going on".
Who are the members and why is it important?
At the moment, 34 states are parties to the Treaty and one (Kyrgyzstan) state has so far signed the treaty, but has yet to ratify it.
Discussing the importance of the Treaty, Dr Lewis said: "Those states all have to co-operate, there is a Consultative Commission, there are many meetings.
"All the information that they gather through the Open Skies Treaty are open [and] the images are shared.
"For many states, this gives them access they wouldn't otherwise have had."
Why is the US threatening to pull out?
Russia has been "messing with the Treaty for the last few years", Dr Lewis said.
She explained that Moscow is trying to create "an independence state approach".
"We have a very difficult situation whereby Russia is definitely challenging the understanding of the Treaty that belongs to all the other states."
How has Russia responded?
The Kremlin maintains that they are not violating the Treaty.
"It maintains that it has every right to do what it's doing and intends to go forward with the Treaty and try to sort it out with all the Treaty restraints," Dr Lewis said.
What happens if America does pull out?
The United States announced its withdrawal from the Treaty in May and this will come to fruition on 22 November.
However, Dr Lewis added that "we don't have long" to try to sort out the situation and the upcoming US Presidential election brings another layer of uncertainty as to the future of Open Skies with regards to the US.
She said: "If the United States does withdraw, we're left with a treaty that has one of its most important players outside."
Dr Lewis added that the US withdrawal "puts all the other NATO countries into a very difficult position".