- India signs deal with Russia despite threat of US sanctions
- In joint address PM Modi says Russia has always stood by India
- This is the third meeting between PM Modi and Mr Putin this year
Here's your 10-point cheat-sheet to this big story:
"In this fast-changing world, our ties have grown at a fast pace. With time, the relations between our countries have gone from strength to strength," PM Modi said in a joint statement with Mr Putin.
"We have agreed to step up cooperation with India in combating the menace of terrorism and drug trafficking," said Mr Putin, who is being accompanied by a high-level delegation comprising Deputy Prime Minister Yuri Borisov, Foreign Minister Sergey Lavrov and Trade and Industry Minister Denis Manturov.
The S-400 is one of the world's most advanced long-range air defence systems. China was the first nation to buy the S-400 missile system in 2014. Russia has already started deliveries of an undisclosed number of the S-400 missile system to Beijing.
New Delhi had already indicated that it will go ahead with the S-400 deal notwithstanding the US sanctions. "As and when the government approves it, delivery (of the missiles) will be in 24 months," Air Chief Marshal BS Dhanoa has told reporters.
Under a new law -- Countering America's Adversaries through Sanctions Act or CAATSA -- the US imposes sanctions on any country that has "significant transactions" with Iran, North Korea or Russia.
India and Russia also signed several agreements in space, nuclear energy and railways. The two leaders reportedly discussed key regional and global issues, including US sanctions on import of Iranian crude oil.
The two countries agreed to cooperate in order to implement new nuclear power projects (NPPs). The pact is expected to be a big boost for the Make in India initiative as it will lead to manufacturing of nuclear fuel assemblies in India.
India is planning to acquire 4 Krivak-class frigates from Russia, two of which will be built in India. Under the $2.5 billion deal, two ships will be built at the Goa shipyard and two others will be acquired from Russia. India already operates six similar frigates.
Russia is one of only two countries with which India holds annual bilateral summits, the other being Japan.
This is the third meeting between PM Modi and Mr Putin this year after the informal meeting in the Russian resort city of Sochi in May and a bilateral on the sidelines of the BRICS Summit in South Africa.