A two-way player is one who has the ability to both pitch and hit, and to a lesser extent field. Thus the player allows a team to fill a roster spot with a pinch-hitter, pinch-runner, position player and pitcher with only one man. They also were very valuable in the early days of baseball, when rosters were much smaller and teams could not afford a large bench, or in leagues with similar limitations. In fact, almost all of the two-way players in major league history date back to the deadball era and before, the exception being the Negro Leagues, where two-way players remained common throughout their existence, because of constraints on roster sizes.
In practice, excluding the Negro Leagues, true two-way players have been extremely rare at the major league level until the end of the 2010s, although there were a few pitchers who were regularly used as pinch-hitters or pinch-runners, and a few position players who were used more than a handful of times as a mystery pitcher. These were not players who normally took both the field and the mound in game situations, however. The last of these was likely Babe Ruth, who took a regular turn on the mound and was also a regular in the field when not pitching in his last season with the Boston Red Sox in 1919; he then became a full-time outfielder after being sold to the New York Yankees, only occasionally taking the mound on special occasions. In the 1950s, a few players like Erv Dusak and twin brothers Eddie O'Brien and Johnny O'Brien were two-way players for a short spell when their teams were toying with the idea of turning them into pitchers. In the 19th Century, two-way players like Bob Caruthers, Tony Mullane and Guy Hecker were more prevalent and successful.
More common are players who are converted from the field to the mound (or much more rarely the other way around) after reaching the major leagues as a position player. However, in almost all cases, these players never take the field again after making the transition to the mound, so are not true two-way players.
Two-way players are common in high school or college ball, however, and in many European leagues. Every year, there are some top picks in the amateur draft who are considered by scouts as a top talent both as a hitter and a pitcher. In all cases, however, the teams which sign them quickly decide whether they will attempt a career as a hitter or as a pitcher, as the conventional thought was until very recently that doing both is simply too difficult at a major league level.
In recent years, Brooks Kieschnick came closest to filling the definition of a two-way player. After washing out of the majors as an outfielder, he resumed pitching in the minors (something he had done successfully in college) and was good enough at it that the Milwaukee Brewers brought him up as a pitcher who would also pinch-hit. During his two seasons in the role, in 2003 and 2004, he only played in the field three times, however, in comparison to 74 pitching appearances.
The question of whether a player with the right skill set could be a successful two-way player in the major leagues today came up again when young Japanese star Shohei Ohtani stated his wish to play in the majors after the 2017 season. He had been a regular (and very good) DH for his team in Japan when not pitching and said openly that he wanted to do the same in Major League Baseball; he had been the first two-way star in Japan in several decades. He achieved that goal to some extent in the majors, as he won the Rookie of the Year Award, largely for his hitting, but also pitched 51 2/3 innings in between various injuries that restricted his time on the mound. Injuries slowed down the experiment over the next few seasons, but he was back at it with a vengeance in 2021, both hitting and pitching, and even batting in his usual #2 spot in the line-up while being used as the starting pitcher. He won the MVP Award and was granted the Commissioner's Historic Achievement Award in recognition of his amazing feats that season. Again, his time as a position player was largely limited to playing DH, as was that of Brendan McKay, a top draft pick who was a two-way player in college and reached the majors in 2019 as a starting pitcher/designated hitter. A truer example is Michael Lorenzen, as he was used both as a relief pitcher and a substitute outfielder in 2019, after having been regularly used as a pinch-hitter in previous seasons.
As part of changes to the rules announced in March 2019 which were supposed to go into effect in 2020, Major League Baseball adopted for the first time a restriction on two-way players, in order to make possible two parallel rule changes: that limiting the number of pitchers on major league rosters; and that limiting the use of mystery pitchers. Thus certain position players were able to earn the designation of "two-way player" if they met minimum usage thresholds both as a pitcher and as a position player during the previous season; this allowed them to be used as pitchers without counting against the limit on the number of pitchers allowed. Until they earned the designation, however, they would only be allowed to pitch in "normal" games if they were listed as pitchers. As there were no limitations on using pitchers in the field or as pinch-hitters or pinch-runners, as this is not considered disruptive, they would have to earn their two-way designation while occupying one of the precious pitching spots on the roster, something that would have been likely to keep the occurrence very infrequent. For example, if the rule had been introduced in 2019, only Ohtani would have qualified for the two-way player designation. Like many other things in baseball, the coronavirus pandemic upended plans to introduce the rule and it was quietly shelved: extra players were added to major league rosters with no positional limits, and most teams decided to use the opportunity to add yet another pitcher.
For a list of two-way players, see Two-way players.
- David Adler: "The new two-way player rule, explained", March 15, 2019. 
- Anthony Castrovince: "How the new 2-way player rule impacts clubs", mlb.com, March 3, 2020. 
- Anthony Castrovince: "Negro Leagues had their own two-way stars", mlb.com, April 26, 2021. 
- Richard Justice: "Ohtani could clear path for 2-way players: Japanese superstar would be 1st successful pitcher-hitter since Ruth", mlb.com, December 4, 2017. 
- Herm Krabbenhoft: "Day-In/Day-Out Double-Duty Diamondeers, 1946-1960", in Baseball Research Journal, SABR, Vol. 49, Nr. 2 (Fall 2020), pp. 91-104.
- Joe Trezza: "Posey, Blackmon highlight All-Two-Way Team: Moreland, Mercer, Bumgarner, Doolittle among players who can rake, hurl", mlb.com, December 6, 2017.