Nemtsov, 55, was shot in the back while walking with his Ukrainian girlfriend in central Moscow on February 28, 2015. A gunman in a car opened fire, prompting immediate speculation that the killing was targeted.
A deputy prime minister in the late 1990s under President Boris Yeltsin, Nemtsov had been one of current President Vladimir Putin's most vocal critics.
Nemtsov was a top official with the Republican Party of Russia/Party of People's Freedom, a liberal opposition group. He was a vocal critic of the Kremlin's handling of the Ukraine crisis.
His life had been threatened on social media weeks before his death, his lawyer said.
Nemtsov was killed two days before he was supposed to attend a large opposition rally in Moscow. Hours before his killing, he gave a radio interview urging people to turn out for the rally.
Putin quickly condemned the killing and expressed his condolences to his family. At the time, Putin's spokesman said he suspected it was a contract hit meant to cause political discord.
5 charged, few answers
Five people have been arrested and charged in connection with Nemtsov's death.
The five suspects, all ethnic Chechens, have been on trial in Moscow since October. All have pleaded not guilty.
Given the current political climate in Russia
, Nemtsov's relatives and former colleagues have expressed concern that the true perpetrators or those who ordered the killing might never be brought to justice.
Other dissidents killed, possibly poisoned
Nemtsov was one of several Kremlin critics to be killed or to be hospitalized with suspected poisoning.
Earlier this month, Nemtsov's friend Vladimir Kara-Murza was in a coma
after a suspected poisoning. Kara-Muzra is part of Open Russia, an organization of anti-Putin activists who calling for open elections, a free press and civil rights reforms.
It was the second time in two years the dissident fell into a coma after a suspected poisoning. Last week, Kara-Muszra left the country to seek further treatment.
His lawyer said the dissident plans to continue to pursue "the restoration of democracy in Russia
Former Russian security agent Alexander Litvinenko was poisoned by a lethal dose of radioactive polonium, his tea spiked in a London hotel during a meeting with two former Russian security service men in 2006.
Litvinenko had blamed the agency for orchestrating a series of apartment bombings in Russia in 1999 that left hundreds dead and led to Russia's invasion of Chechnya later that year.
Anna Politkovskaya, a journalist critical of Russia's war in Chechnya, was gunned down at the entrance to her Moscow apartment in 2006.
The Kremlin has staunchly denied accusations that it or its agents are targeting political opponents or had anything to do with the deaths.