Here's your 10-point cheatsheet in this big story:
The deal for S-400 units is worth Rs 35,000 crore. Putin is scheduled to hand over the model of an S-400 air defence system to PM Modi.
The first units of the system, once described by then Air Chief BS Dhanoa as a "booster dose" are reportedly being deployed in India. Indian personnel have trained in Russia to use the system.
The missile system is likely to be first deployed at a location close to the western border, from where it can tackle threats from both Pakistan and China.
Parts of the air defence system have started reaching India. Reports say the equipment is being brought to India through both sea and air routes.
The deal signed in 2018 has caused some strain on India-US ties with the US threatening sanctions under the Countering America's Adversaries Through Sanctions Act (CAATSA).
Russia today accused the US of undermining the India-Russia cooperation. "We witnessed attempts on the part of the United States to undermine this cooperation and to make India obey the American orders, to follow the American vision of how this region should be developed," Russian Foreign Minister Sergey Lavrov told news agency ANI.
India pushed ahead despite the threat of sanctions. A similar purchase by NATO ally Turkey prompted the US to ban Ankara from its advanced F-35 fighter jet program under CAATSA.
It is a sovereign decision, the government told parliament recently. "Government takes sovereign decisions based on threat perception, operational and technological aspects to keep the Armed Forces in a state of readiness to meet the entire spectrum of security challenges," said junior Defence Minister Ajay Bhatt.
The minister also said S-400 Missile is a "potent system in terms of its operational capability to provide continuous and effective air defence system to a very large area".
Russia has long been a key arms supplier to India. The long-range S-400 ground-to-air missile defence system is one of their most high-profile current contracts.