The 2016 Summer Olympic Games in Rio will feature some of the world's biggest athletes. If all goes according to plan, international superstars Venus and Serena Williams, Rory McIlroy, Michael Phelps and Usain Bolt will all be there.

And yet, there are a few big names who decidedly will not be. The jury's still out on LeBron James, but the Olympics will definitely go down in Rio without Chris Paul and Lionel Messi.

There are a variety of reasons for the absences—injury, personal decision or even failure to make the team, to name a few. Whatever the cause, these 15 stars will be missed.

On the Fence

Team USA basketball announced 30 finalists for its 12 Olympic roster spots in January. Several of those players have already removed themselves from the running, and more could be on the way. Here are a few superstar NBA players who are serious "maybes" for the Olympics: 

• Cleveland Cavaliers forward LeBron James: still undecided 
• Indiana Pacers forward Paul George: See above 
• New York Knicks forward Carmelo Anthony: samesies

Elsewhere, the entire Russian track and field team is up in the air as a result of its ban from international competition.

Ben Simmons

LSU's Ben Simmons is presumably going to go pretty high in June's NBA draft, and that's where he wants to focus.

The 19-year-old forward chose not to play for his native Australia in the upcoming Olympic Games.

His agent, Rich Paul, said in a statement (via Chris Broussard of ESPN.com), "Australian Olympic Team training and competition takes place at the same time as the various NBA summer development programs, and therefore, Ben will look forward to his national team participation at some time later in his career."

Simmons averaged 19.2 points, 11.8 rebounds and 4.8 assists per game in one season with the Tigers.

Javier 'Chicharito' Hernandez

The Copa America Centenario and the Olympics are both going down this summer. World Cup qualifying matches are also taking place into the fall. So it's a busy year for many international teams, and some players must decide which tournaments they can realistically play in.

For instance, rising Mexican star Javier "Chicharito" Hernandez will go with Option A and forgo the Olympics to play in the Copa America. The 27-year-old striker has performed well for Bayer Leverkusen in 2015-16, netting 17 Bundesliga goals. 

Hernandez explained, "The [Mexican] federation wants me to go to Copa America and I decided to go to Copa America. Not because I don't want to go to the Olympics. It's because it's the best for...my club, for me and for the federation," according to Kevin Baxter of the Los Angeles Times.

John Isner

Serena and Venus Williams are all set to go for Rio, but the top American male won't be there.

John Isner is ranked No. 16 in the world, but according to Doug Roberson of the Atlanta Journal-Constitution, the 31-year-old "said he wants to rest that week as he focuses on the bigger tournaments in the hard-court season."

Per Roberson, Isner added, "It's a decision I didn't take lightly."

Part of the decision also reportedly had to do with the timing of the BB&T Atlanta Open, a tournament Isner has won three times. It will be played in late July and early August, right before Olympic tennis is set to kick off.

Erin McLeod

Canadian goalkeeper Erin McLeod has played in two Olympic Games, 2008 and 2012, and helped her team to a bronze medal in London. She has 43 shutouts in 115 international caps.

Unfortunately, an ACL injury will force the 33-year-old to miss the competition in Rio.

John Herdman said, "Erin is an incredible athlete, and her absence in the lead-up to and during the Rio Olympics will be felt. She really puts her whole self into playing for Canada and has been a driving force in every area, including playing a key role on the off-field leadership of the team," per the Canadian Press (via the Globe and Mail).

Kristaps Porzingis

Kristaps Porzingis made a huge splash in the NBA in 2015-16 as one of the best rookies of his class. And yet, the 20-year-old New York Knicks power forward has opted not to represent his native Latvia in the Summer Games.

According to Ian Begley of ESPN.com, Porzingis said, "My heart is saying you should go and play for your country, you should represent your country. At the same time, with my head, I'm understanding this is a big offseason for me and I really want to work individually."

The rookie averaged 14.3 points and 7.3 rebounds in 72 games in 2015-16.

Adam Scott

Golf is making its return to the Olympic Games in 2016 for the first time since 1904, but not everyone is on board.

For instance, the 2013 Masters champion, Australian Adam Scott, chose not to compete. In April, the world's No. 7 announced in a statement (via Alex Myers of Golf Digest), "My decision has been taken as a result of an extremely busy playing schedule around the time of the Olympics, and other commitments, both personal and professional."

Some have expressed displeasure over Scott's choice, but he's not the only one to opt out. Other pro golfers who have elected not to compete include Vijay Singh of Fiji and Australian Marc Leishman.

Dennis Kimetto

The name Dennis Kimetto might not sound familiar to those not up to date on the international marathon scene, but the Kenyan distance runner is the world record holder at the 26.2-mile distance.

Kimetto set the record with a time of 2:02:57 at the Berlin Marathon in 2014.

Yet, according to Isaack Omulo of Reuters, Kimetto has failed to perform well enough recently to earn a spot on the Kenyan national team, which will include three men and three women.

The men's team will feature the first- and second-place finishers at the 2016 London Marathon (Eliud Kipchoge and Stanley Biwott) and the fourth-place Boston Marathon finisher (Wesley Korir).

Anthony Davis

New Orleans Pelicans center Anthony Davis played in his first Olympic Games in 2012 and won a gold medal with Team USA in London.

In March, however, season-ending shoulder surgery made a repeat appearance in Rio an impossibility.

Per Kelly Dwyer of Yahoo Sports, Davis said, "It's definitely tough. Twenty three years old ... [I've got] a couple more Olympics, maybe? It's definitely a tough situation. I love USA Basketball [...] It was definitely a tough decision, but I think everybody understands where health is more important than [being] on the court."

Of the 30 finalists for Team USA announced in January, four others are centers: DeMarcus Cousins, Andre Drummond, Dwight Howard and DeAndre Jordan.

Sydney Leroux

Sydney Leroux is an up-and-coming star on the international soccer scene. The forward scored her first Olympic goal at the 2012 Games in London, where the U.S. won gold, and she was a part of the instantly legendary 2015 World Cup championship team as well.

Leroux will miss the Olympics in Rio, however, due to a pregnancy. Nick Martin of the Washington Post noted she would not have been a "shoo-in" on the team due to recent injuries, but regardless, the possibility ended with her announcement in January.

Forward Amy Rodriguez will also miss the Games due to pregnancy, and longtime U.S. midfielder Heather O'Reilly was left off the qualifying roster by coach Jill Ellis.

Blake Griffin

Though he was on the list of 30 finalists for Team USA basketball, Los Angeles Clippers forward Blake Griffin will not be competing in Rio.

A quad injury ended his 2015-16 NBA season, and shortly thereafter, the Clippers announced Griffin would officially not participate with Team USA.

According to the official release from the team on NBA.com, "Griffin underwent a bone marrow procedure on his left quad tendon on April 27 and has already begun the rehabilitation process with the expectation that he will be ready for the start of the 2016-17 NBA season."

Candace Parker

Los Angeles Sparks star Candace Parker has won two Olympic gold medals but will not have the opportunity to compete for a third in Rio. The 30-year-old was left off the roster in April.

She told the Associated Press (via USA Today), "I was surprised and disappointed. Having gone to [the] last two Olympics, I know what it means to represent the USA. I wish everybody on the team good luck. The USA is going to win a sixth gold medal."

Playing in 16 games of the 2015 WNBA season, Parker averaged 19.4 points, 10.1 rebounds and 6.3 assists per game.

Cristiano Ronaldo

One of the world's biggest stars, Cristiano Ronaldo of Portugal and Real Madrid, won't compete in the Olympics, according to Alberto Pinero of Goal.com.

Pinero cited "sources close to Ronaldo" and wrote in April, "Given the 31-year-old has played 50 games for Madrid in four of his last five seasons and is likely to do so again by the end of the current campaign, adding another tournament to his schedule has been deemed unrealistic."

Argentina and Portugal are in the same group for the Olympic tournament, but alas, Messi vs. Ronaldo is not happening.

Kobe Bryant

Kobe Bryant announced his NBA retirement back in November and also removed himself from Team USA Olympic consideration in January.

Bryant is 37 years old, and his place on the team was by no means assured. Still, he was one of the first big superstars to take himself out of the running.

Per Tim Reynolds of NBA.com, the two-time Olympic gold medalist said, "Since my retirement announcement, I'm able to watch these guys in a different light. I've come to terms with the fact that they are the future of this game. These are the guys who deserve the spots in Rio."

Chris Paul

Even before a hand injury ended his NBA postseason, Los Angeles Clippers point guard Chris Paul removed himself from contention for the U.S. men's Olympic basketball team.

Paul helped Team USA win gold in 2008 and 2012, but in March, he told Lee Jenkins of SI.com, "I feel my body telling me that I could use the time."

Paul is 31 years old and still seeking his first NBA title. He shouldered a heavy load in the 2015-16 season with teammate Blake Griffin missing significant time, yet he still averaged 19.5 points and 10.0 assists per game.

Lionel Messi

Argentina could have up to three players over the age of 23 on its Olympic roster, but Barcelona superstar Lionel Messi will not be one of them. He will play in the Copa America in June, however. 

Argentina coach Gerardo Martino told Clarin (via ESPNFC), "To expose Leo to playing a new competition, after all he has done, seems to me to be too much. We would again be damaging ourselves, considering what is ahead in the second half of the year."

Messi's Barca teammate Neymar, however, will do the opposite. He will not play in the Copa America but will compete for the host Brazil in the Olympics.