FidoNet is a worldwide computer network that is used to transfer messages between bulletin board systems (BBSes). It uses a store-and-forward system to exchange private (email) and public (echo) messages between the BBSes that are members of the network, as well as other files and protocols in some cases.

The FidoNet system was based on one or more interacting programs. The tosser process interacts with the BBS messages directly and is the main process that must to be ported to support other BBS software. This greatly eased porting. The FidoNet mailer process was one of the few message networks that was widely supported by almost all BBS software, as well as a number of non-BBS services. This modular construction also allowed FidoNet to easily upgrade to new data compression systems, which was important in an era using modem-based communications over telephone links with high long-distance calling charges.

The rapid improvement in modem speeds during the early 1990s, combined with the rapid decrease in price of computer systems and storage, made BBSes increasingly popular. By the mid-1990s there were almost 40,000 FidoNet systems in operation, and it was possible to communicate with millions of users around the world. Only UUCPNET came close in terms of breadth or numbers; FidoNet's user base far surpassed other networks.

The broad availability of low-cost Internet connections starting in the mid-1990s lessened the need for FidoNet's store-and-forward system, as any system in the world could be reached for equal cost. Direct dialing into local BBS systems rapidly declined. Although FidoNet has shrunk considerably since the late 1990s, it has remained in use even today, despite internet connectivity becoming universally available.

Around Christmas 1983, Tom Jennings started work on a new MS-DOS–hosted bulletin board system that would later emerge as the BBS software called Fido. Jennings set up the system in San Francisco in early 1984. Another early user was John Madil, who was trying to set up a similar system in Baltimore on his Rainbow 100. Fido BBS Software started spreading to new systems, and Jennings eventually started keeping an informal list of their phone numbers, with Jennings becoming #1 and Madil #2.

Jennings released the first version of the FidoNet software in June 1984. In early 1985, he wrote a document explaining the operations of the FidoNet, along with a short portion on the history of the system. In this version, FidoNet was developed as a way to exchange mail between the first two Fido BBS systems, Jennings' and Madil's, to “see if it could be done, merely for the fun of it”. This was first supported in Fido BBS V7, “sometime in June 84 or so”.

In early 1984, Ben Baker was planning on starting a BBS for the newly forming computer club at the McDonnell Douglas automotive division in St. Louis. Baker was part of the CP/M special interest group (SIG) within the club. He intended to use the seminal, CP/M-hosted, CBBS system, and went looking for a machine to run it on. The club's president told Baker that DEC would be giving them a Rainbow 100 computer on an indefinite loan, so he made plans to move the CBBS onto this machine. The Rainbow contained two processors, an Intel 8088 and a Zilog Z80, allowing it to run both MS-DOS and CP/M, with the BBS running on the latter. When the machine arrived, they learned that the Z80 side had no access to the I/O ports, so CBBS could not communicate with a modem. While searching for software that would run on the MS-DOS side of the system, Baker learned of Fido through Madil.

The Fido software required changes to the serial drivers to work properly on the Rainbow. A porting effort started, involving Jennings, Madil and Baker. This caused all involved to rack up considerable long distance charges as they all called each other during development, or called into each other's BBSes to leave email. During one such call “in May or early June”, Baker and Jennings discussed how great it would be if the BBS systems could call each other automatically, exchanging mail and files between them. This would allow them to compose mail on their local machines, and then deliver it quickly, as opposed to calling in and typing the message in while on a long-distance telephone connection. Jennings responded by calling into Baker's system that night and uploading a new version of the software consisting of three files: FIDO_DECV6 (the new version of the BBS program itself), FIDONET, and NODELIST.BBS. The new version of FIDO BBS had a timer that caused it to exit at a specified time, normally at night, and as it exited it would run the separate FIDONET program. NODELIST was the list of Fido BBS systems, which Jennings had already been compiling.

The FIDONET program was what later became known as a mailer. FIDO was modified to use a previously unused numeric field in the message headers to store a node number for the machine the message should be delivered to. When FIDONET ran, it would search through the email database for any messages with a number in this field. FIDONET collected all of the messages for a particular node number into a file known as a message packet. After all the packets were generated, one for each node, the FIDONET program would look up the destination node's phone number in NODELIST.BBS, and call the remote system. Provided that FIDONET was running on that system, the two systems would handshake and, if this succeeded, the calling system would upload its packet, download a return packet if there was one, and disconnect. FIDONET would then unpack any received packet, and place the received messages into the local system's storage, and move onto the next packet. When there were no remaining packet, it would exit, and run the FIDO BBS program.

In order to lower long distance charges, the mail exchanges were timed to run late at night, normally 4 AM. This would later be known as national mail hour, and, later still, as Zone Mail Hour.

FidoNet started in 1984 and listed 100 nodes by the end of that year. Steady growth continued through the 1980s, but a combination of factors led to rapid growth after 1988. These included faster and less expensive modems, and rapidly declining costs of hard drives and computer systems in general. By April 1993 the FidoNet nodelist contained over 20,000 systems. At that time it was estimated that each node had, on average, about 200 active users. Of these 4 million users in total, 2 million users commonly used echomail, the shared public forums, while about 200,000 used the private netmail system. Fidonet at its peak, listed approximately 39,000 systems in the weekly nodelist.

As modems reached speeds of 28.8 kbit/s, the overhead of the TCP/IP protocols were no longer so egregious and dial-up Internet became increasingly common. By 1995 the bulletin board market was reeling as users abandoned local BBS systems in favor of larger sites and web pages, which could be accessed worldwide for the same cost as accessing a local BBS system. This also made FidoNet less expensive to implement, because FidoNet transfers could be delivered over the Internet as well, at little or marginal cost. But this seriously diluted the entire purpose of the store-and-forward model, which had been built up specifically to address a long-distance cost problem that no longer existed.

The FidoNet nodelist started shrinking, especially in areas with widespread availability of internet connections. This downward trend continues, but has leveled out at approximately 1,500 nodes. FidoNet remains popular in areas where Internet access is difficult to come by, or expensive.

There is now (~2014) a retro movement which is resulting in a slow increase in internet connected BBS and nodes. Telnet, Web Telnet, and SSH are being used between systems. This means you can telnet to many BBS worldwide as cheaply as ones next door. Also Usenet and internet mail has been added to many systems, along with support for long file names to many newer versions of BBS software, some being free-ware, resulting in their increasing use.