(CNN) -

Here's a look at what you need to know about lockouts and strikes in professional sports. A lockout is imposed by management or the owners. A strike is initiated by the employees or players.

National Football League (NFL) Lockouts & Strikes: There have been six NFL work stoppages.

March 12-July 25, 2011 - The 136-day lockout, the longest work stoppage in NFL history.

Timeline of NFL Lockouts & Strikes: July 3-July 15, 1968 - The NFL Players Association votes to strike, and the NFL owners counter by locking out the players during training camp. The two sides come to an agreement regarding pension benefits, resulting in the first NFL collective bargaining agreement.

July 13-August 3, 1970 - In response to NFL veterans being locked out of training camp, the NFL Players Association votes to strike. The second NFL work stoppage ends with a four-year, $19.1 million deal.

July 1-August 10, 1974 - The 41-day strike ends when the veteran NFL players report to training camp without a new agreement.

September 21-November 16, 1982 - The 57-day strike is over revenue sharing. Play resumed on November 21.

September 22-October 15, 1987 - A 24-day strike. After the NFL players go on strike over free agency they are supplanted by replacement players.

March 8, 2006 - The collective bargaining agreement is extended through the 2011 season. Under the agreement, of the $9 billion revenue stream, NFL owners take $1 billion off the top for expenses. After that, the players get about 60%.

May 20, 2008 - The owners vote to end the collective bargaining agreement after the 2010 season.

February 18, 2011 - The NFL and the NFL Players Association (NFLPA) enter into federal mediation. George Cohen is the Director of the Federal Mediation and Conciliation Service (FMCS), an independent government agency handling the mediation.

March 3, 2011 - The existing contract between the NFL and the players expires at 11:59 pm EST.

March 4, 2011 - The negotiating deadline is extended by one week.

March 11, 2011 - Negotiations to draft a new collective bargaining agreement break down. At 4:40 p.m. EST, the players' union walks out on negotiations.

March 11, 2011 - After the NFLPA walks out on negotiations, the players decertify the union so that they can file a class-action antitrust lawsuit against the National Football League (Brady v NFL). Quarterbacks Tom Brady, Drew Brees, Peyton Manning are three of 11 football players who file the lawsuit on behalf of the players against the NFL.

March 12, 2011 - NFL lockout begins at midnight ET.

April 25, 2011 - U.S. District Judge in St. Paul, Minnesota Susan Richard Nelson rules in favor of the players, ordering the owners to end the lockout.

April 27, 2011 - Judge Nelson denies the NFL's appeal.

April 29, 2011 - The lockout goes back into effect as the league's appeal for a temporary stay of the ruling is granted by the 8th U.S. Circuit Court of Appeals.

July 21, 2011 - NFL owners vote 31-0 (Oakland Raiders abstain) to ratify the proposed 10-year collective bargaining agreement.

July 25, 2011 - The NFL players unanimously vote to ratify the collective bargaining agreement, effectively ending the 136-day lockout.

August 4, 2011 - NFL players ratify a new collective bargaining agreement, formally ending the labor dispute.

National Basketball Association (NBA) Lockouts: July 1-September 18, 1995 - The 80-day lockout revolves around the players dissatisfaction with their union and salaries.

July 1, 1998-January 6, 1999 - The 191-day lockout is over collective bargaining.

July 1-December 8, 2011 - The 161-day lockout ends when NBA players and owners agree to a 10-year collective bargaining deal.

Timeline of NBA Lockouts: June 21, 1995 - Owners and the players' union agree to a six-year deal.

July 1, 1995 - NBA Commissioner David Stern announces a lockout after players reject the agreement reached by the owners and the players' union. The players claim they weren't fairly represented by the union. Simon Gourdine is the Executive Director and leader of the players' union.

August 8, 1995 - The NBA players and the owners reach an agreement.

September 12, 1995 - The players accept the terms of the agreement, by a vote of 226-134.

September 13, 1995 - Representatives of the players' union approves the agreement in a 25-2 vote.

September 15, 1995 - Team owners ratify the contract in a 24-5 vote.

September 18, 1995 - The NBA owners and players' union agree to a six-year, $5 billion collective bargaining agreement.

March 23, 1998 - The NBA owners vote to reopen the 1995 collective bargaining agreement, 27 to 2.

July 1, 1998 - The NBA owners lockout the players.

January 6, 1999 - The NBA players and owners reach a tentative agreement.

January 6, 1999 - The NBA players consent to the terms of the agreement by a vote of 179-5.

January 7, 1999 - The lockout officially ends when all 29 NBA owners unanimously accept the agreement.

June 30, 2011 - The current collective bargaining agreement expires.

July 1, 2011 - The NBA begins a collective lockout of its players. The lockout will continue until a new collective bargaining agreement is reached with the National Basketball Players Association.

August 2, 2011 - The NBA files two claims against the National Basketball Players Association: an unfair labor practice charge before the National Labor Relations Board and a lawsuit in New York federal district court.

October 18, 2011 - NBA players and owners meet with federal mediator George Cohen.

November 14, 2011 - The NBA players reject the league's latest labor proposal and announce that the union will disband and an antitrust lawsuit will be filed against the NBA.

November 15, 2011 - The NBA announces that all games through December 15, a total of 324 games, are canceled. Hours later, NBA players including Carmelo Anthony and Kevin Durant file class-action antitrust lawsuits against the NBA in at least two states.

November 26, 2011 - NBA players and owners reach a tentative agreement, effectively ending the lockout. The NBA season is scheduled to begin on December 25, 2011.

December 8, 2011 - The lockout officially ends when NBA players and owners ratify a 10-year collective bargaining agreement.

National Hockey League (NHL) Lockouts & Strikes: There have been four work stoppages in the NHL since 1992, as a result of 1 strike and 3 lockouts.

2004-2005 - First time that a North American professional sports league loses an entire season due to a labor dispute.

Timeline of NHL Lockouts & Strikes: April 1992 - NHL players go on strike, resulting in the postponement of 30 games.

October 1, 1994-January 11, 1995 - 468 games are canceled as a result of the 103-day lockout.

September 15, 2004 - The existing collective bargaining agreement expires at midnight EST.

September 16, 2004 - The NHL owners lockout the players.

February 16, 2005 - 2004-2005 hockey season is canceled due to lockout.

March 24, 2005 - The 2005 NHL draft is canceled.

July 13, 2005 - After more than 24 hours straight of talks, a tentative deal is made.

July 22, 2005 - The lockout ends when the NHL and the NHL Players' Association finalize a 6-year collective bargaining agreement. The deal includes a team salary cap system, the main source of disagreement, with a limit on total league spending for players.

June 2010 - The NHL Players' Association extends the existing collective bargaining agreement through the 2011-2012 season.

September 15, 2012 - The NHL imposes a lockout at midnight EST after the collective bargaining agreement expires.

December 10, 2012 - The National Hockey League announces the cancellation of the 2012-13 regular-season schedule through December 30. A total of 526 regular-season games were scheduled for October 11 through December 30.

January 6, 2013 - After a 16-hour negotiating session, the National Hockey League and the NHL Players' Association reach a tentative agreement.

January 9, 2013 - The National Hockey League owners vote unanimously to ratify the new 10-year collective bargaining agreement.

January 12, 2013 - The NHL and the NHL Players' Association sign a Memorandum of Understanding regarding the new 10-year collective bargaining agreement, officially ending the lockout. The truncated regular season begins January 19 and ends April 27.

Major League Baseball (MLB) Lockouts & Strikes: There have been 8 work stoppages in Major League Baseball history, as a result of 5 strikes and 3 lockouts.

1994-1995 - The most recent and longest work stoppage, due to a strike.

Timeline of MLB Lockouts & Strikes: April 1-13, 1972 - Strike concerning player pensions and binding arbitration. Lasts 13 days, 86 games missed.

February 8-25, 1973 - Lockout concerning binding arbitration for salary disputes. Lasts 17 days, 0 games missed.

March 1-17, 1976 - Lockout concerning free agency and re-entry draft. Lasts 17 days, 0 games missed.

April 1-8, 1980 - Strike concerning free-agent compensation. Lasts 8 days, 0 games missed.

June 12-July 31, 1981 - Strike ordered by the court, concerning team owners free-agent compensation. Lasts 50 days, 712 games missed.

August 6-7, 1985 - Strike concerning owner contributions to player pension plan and binding arbitration. Lasts 2 days, 0 games missed.

February 15-March 18, 1990 - Lockout concerning revenue sharing, salary arbitration and the salary cap. Lasts 32 days, 0 games missed.

August 12, 1994-March 31, 1995 - Strike concerning the salary cap. Lasts 232 days, 938 games missed including the entire 1994 playoff and world series schedule.

August 30, 2002 - Hours before a strike deadline set by the players, negotiators reach an agreement. Settlement was ratified by Owners 29-1, with New York Yankees being the only opposed to it. The deal marks the first time in the last 30 years that a collective bargaining agreement between players and owners was reached without a work stoppage.

November 22, 2011 - MLB players and owners sign a new agreement; which includes testing players' blood for human growth hormone, expanding the playoffs from eight teams to ten and moving the Houston Astros to the American League in 2013.

December 2016 - The current collective bargaining agreement is set to expire.