19th Century Era  1876-1900
The highest level of professional baseball consisted of the National League, American Association (1882-91), as well as, briefly, the Union Association (1884) and Players League (1890).  Foul balls were not strikes during this period. A base on balls once required three balls, then nine, then eight, then seven. then six, then back to seven before finally landing at four.  Pitching distances were moved from 45 feet to 50 feet to 60.5 feet.  For a while, pitchers were prohibited to throw over the shoulder and batters were permitted to call for high or low strikes to be thrown to them.  Until 1893, batters were allowed to use a bat that was flat on one side.  Home plate was shaped like a diamond instead of a pentagon.  Starting pitchers completed their games 90% of the time.  Many other rules were born or evolved.

Dead Ball Era  1901-1919
The American League joined the National League as a top level professional baseball circuit.  A third group, the Federal League also existed for a short period (1914-15).  Balls were used as long as possible in play.  Runs were scarce and homeruns were far and few between.  Pitchers were allowed to use altered baseballs and trick pitches.  The "scientific" method (or what is referred today as "small ball") was the primary method of play - advancing runners, stealing bases, playing for one run, etc.  Starting pitchers completed their games two-thirds of the time.  Issues such as gambling and game fixing came to head in with the throwing of the 1919 World Series by several members of the Chicago White Sox.

Lively Ball Era  1920-1941
In this time, a cork-and-rubber-center ball, replacing a rubber-core ball, was now consistently used by both the National and American Leagues.  Pitchers, sans a handful of grand-fathered cases, were no longer permitted to alter baseballs or use trick pitches.  A fresh ball was used in play at all times.  During this period, on average, nearly ten runs were scored per game.  Batting average and homerun records were shattered.  In 1930, the National League's batting average was over .300.  Starting pitchers completed their games 47% of the time.  In 1935, "night" baseball was introduced.  For the most part, clubs traveled from city to city by train.  Teams began broadcasting their games on the radio and later television. 

Also, in 1920, the Negro Leagues were formed in an answer to racial discrimination in the major leagues.

Integration Era  1942-1960
It is significant to note that, during the first half of the 1940's, over one hundred major league players were involved in World War II.  As a result, the level of play was somewhat diluted.  After the war, in 1947, Jackie Robinson became the first black to play major league baseball since 1884.  After Robinson's debut, each team in both leagues signed their first black player.  Although, on average, most teams did not have a black player receiving regular playing time until 1954 and three teams waited until 10 years after Robinson to sign their first black.  Integration by Major League Baseball subsequently led to the demise of the Negro Leagues.  Offense was slightly down from the Lively Ball Era; although, homeruns were still on the rise.  Starting pitchers completed their games 37% of the time.  

Expansion Era  1961-1976  
An enlarged strike zone and expansion from 16 to 20, and then 20 to 24, teams gave cause for a reduction in the average offensive output - as compared to the Lively Ball and Integration Eras.  In 1965, a player draft was implemented where teams would select, in reverse order of their finish in the standings, amateur talent to sign.  In 1969, each league split into two divisions and then employed a season-end League Championship Series to determine their champion.  In response to the shifting of dominance from offense towards pitching, in 1969, the pitcher's mound was lowered; and, 1973, the American League adopted the use of the Designated Hitter (for pitchers).  Starting pitchers completed their games 26% of the time in this period.  

Free Agency Era  1977- 1993
Players bargained for the right to "free agency" after their sixth major league season; and, this resulted in increased player movement and skyrocketing player salaries.  During this seventeen year era, fourteen different franchises won World Championships.  On average, runs per game during this period rose back to the levels found during the Integration Era. At times, as many as one-third of the teams in the majors used artificial turf fields which lead to an increased emphasis on scoring via doubles, triples, and stolen bases (rather than a reliance mostly on the homerun).  Starting pitchers completed their games 15% of the time.  

Long Ball Era  1994- 2005
Homeruns and strikeouts increased immensely during this era.  This has been attributed to several factors which include the introduction of several new ballparks which were favorable to hitters, the acceptance of strength training by players, the reluctance of pitchers to throw inside, and a shift in batting technique by many coupled with the use of bats more conducive to driving the ball.  A rapid increase in the number of relief pitchers used per game occurred.  Starting pitchers completed their games only eight percent of the time.  At the start of this era, leagues began using a three division format and instituted a season-end League Divisional Series (including a Wild Card team) to determine who should play in the League Championship Series.  Select games were played outside the Unites States or Canada for the first time.  In 1997, the leagues began inter-league regular season play.