A play-by-mail game (also known as a PBM game, PBEM game, turn-based game, turn based distance game, or an interactive strategy game.) is a game played through postal mail, email, or other digital media. Correspondence chess and Go were among the first PBM games. Diplomacy has been played by mail since 1963, introducing a multi-player aspect to PBM games. Flying Buffalo Inc. pioneered the first commercially available PBM game in 1970. A small number of PBM companies followed in the 1970s, with an explosion of hundreds of startup PBM companies in the 1980s at the peak of PBM gaming popularity, many of them small hobby companies—more than 90 percent of which eventually folded. A number of independent PBM magazines also started in the 1980s, including The Nuts & Bolts of PBM, Gaming Universal, Paper Mayhem and Flagship. These magazines eventually went out of print, replaced in the 21st century by the online PBM journal Suspense and Decision.

Play-by-mail games (which became known as "turn-based games" in the digital age) have a number of advantages and disadvantages compared to other kinds of gaming. PBM games have wide ranges for turn lengths. Some games allow turnaround times of a day or less—even hourly. Other games structure multiple days or weeks for players to consider moves or turns and players never run out of opponents to face. If desired, some PBM games can be played for years. Additionally, the complexity of PBM games can be far beyond that allowed by a board game in an afternoon, and pit players against live opponents in these conditions—a challenge some players enjoy. PBM games allow the number of opponents or teams in the dozens—with some previous examples over a thousand players. PBM games also allow gamers to interact with others globally. Games with low turn costs compare well with expensive board or video games. Drawbacks include the price for some PBM games with high setup and/or turn costs, and the lack of the ability for face-to-face roleplaying. Additionally, for some players, certain games can be overly complex, and delays in turn processing can be a negative.

Play-by-mail games are multifaceted. In their earliest form they involved two players mailing each other directly by postal mail, such as in correspondence chess. Multi-player games, such as Diplomacy or more complex games available today, involve a game master who receives and processes orders and adjudicates turn results for players. These games also introduced the element of diplomacy in which participants can discuss gameplay with each other, strategize, and form alliances. In the 1970s and 1980s, some games involved turn results adjudicated completely by humans. Over time, partial or complete turn adjudication by computer became the norm. Games also involve open- and closed-end variants. Open-ended games do not normally end and players can develop their positions to the fullest extent possible; in closed-end games, players pursue victory conditions until a game conclusion. PBM games enable players to explore a diverse array of roles, such as characters in fantasy or medieval settings, space opera, inner city gangs, or more unusual ones such as assuming the role of a microorganism or a monster.

The earliest play-by-mail games developed as a way for geographically separated gamers to compete with each other using postal mail. Chess and Go are among the oldest examples of this. In these two-player games, players sent moves directly to each other. Multi-player games emerged later: Diplomacy is an early example of this type, emerging in 1963, in which a central game master manages the game, receiving moves and publishing adjudications. According to Shannon Appelcline, there was some PBM play in the 1960s, but not much. For example, some wargamers began playing Stalingrad by mail in this period.

In the early 1970s, in the United States, Rick Loomis, of Flying Buffalo Inc., began a number of multi-player play-by-mail games; these included games such as Nuclear Destruction, which launched in 1970. This began the professional PBM industry in the United States. Professional game moderation started in 1971 at Flying Buffalo which added games such as Battleplan, Heroic Fantasy, Starweb, and others, which by the late 1980s were all computer moderated.

"Rick Loomis is generally recognized as the founder of the PBM industry."

For approximately five years, Flying Buffalo was the single dominant company in the US PBM industry until Schubel & Son entered the field in roughly 1976 with the human-moderated Tribes of Crane. Schubel & Son introduced fee structure innovations which allowed players to pay for additional options or special actions outside of the rules. For players with larger bankrolls, this provided advantages and the ability to game the system. The next big entrant was Superior Simulations with its game Empyrean Challenge in 1978. Reviewer Jim Townsend asserted that it was "the most complex game system on Earth" with some large position turn results 1,000 pages in length.

Chris Harvey started the commercial PBM industry in the United Kingdom with a company called ICBM. After Harvey played Flying Buffalo's Nuclear Destruction game in the United States in approximately 1971, Rick Loomis suggested that he run the game in the UK with Flying Buffalo providing the computer moderation. ICBM Games led the industry in the UK as a result of this proxy method of publishing Flying Buffalo's PBM games, along with KJC games and Mitregames.