Mario & Luigi (also known as Mario & Luigi RPG in Japanese regions) is a series of role-playing video games published by Nintendo and originally developed by AlphaDream prior to their bankruptcy. The series is a spin-off from Nintendo's trademark Super Mario series and stars the titular characters Mario and Luigi. The games' stories typically follow the two exploring locales unique to the series on a quest to defeat an antagonist, most of the time original. It began in 2003 on the Game Boy Advance with Mario & Luigi: Superstar Saga, with the latest original installment being Mario & Luigi: Brothership, which released for the Nintendo Switch on November 7, 2024. Two titles in the series, Superstar Saga and Bowser's Inside Story, were remade for the Nintendo 3DS with extra content, with the latter being the final game in the series developed by AlphaDream before they declared bankruptcy in 2019.

The gameplay of the Mario & Luigi series consists of mostly role-playing elements; however, the Mario & Luigi games do differ from most other RPGs in its focus on controlling Mario and Luigi simultaneously. During overworld sections, the player controls Mario's movement with Luigi following closely. Mario and Luigi's other actions are controlled individually with the A (Mario) and B (Luigi) buttons respectively in both the overworld and battle sections. These controls are also used to explore the overworld to solve puzzles and find collectibles using various special moves that often have Mario and Luigi working together. Like many other RPG series, the Mario & Luigi series contains a traditional turn-based battle system. It also uses Action Commands, similarly to the Paper Mario franchise. These actions consist of Mario's and Luigi's signature jumps, as well as hammer attacks. Unique to the series is the use of real-time commands while an enemy is attacking so that if the player uses successful timing, they can completely avoid an attack or even do damage to the enemy instead of getting hit. This mechanism encourages the player to learn enemy's attack patterns and proper timing. Some titles in the series feature additional characters alongside Mario and Luigi; these include Baby Mario and Baby Luigi in Partners in Time, Bowser in Bowser’s Inside Story, and Paper Mario in Paper Jam.

"Because I don't want to end up in a situation where my mind is too clouded by nostalgia for old games, and I can't focus on how to best explore new avenues and new ways of bringing fun and surprising experiences to people."

AlphaDream was founded in 2000 by former employees from Square including Chihiro Fujioka, director of Super Mario RPG, and Tetsuo Mizuno, Square's second president. Square had previously developed Super Mario RPG, the first role-playing game (RPG) starring characters from the Mario series. After a number of smaller games, Mario & Luigi: Superstar Saga was their first game to be released outside of Japan.

The Mario & Luigi series took inspiration from Super Mario RPG when it came to turn-based combat. Yoshihiko Maekawa, a producer at AlphaDream who also co-directed Super Mario RPG, noted how the latter was inspired by a children's toy in Japan where the player had to press buttons in time with music. From there he conceptualized turn-based mechanics that blended action with general RPG gameplay, where the player had to make timed button presses to be more successful. It was there the developers knew they were creating an RPG game that had a Mario feel. Transitioning to the Mario & Luigi series, AlphaDream revisited the timed button pressing combat system, making additions for the first game in the series, Mario & Luigi: Superstar Saga. Each game following, they experimented more with the buttons available, making further additions as the series progressed.

When it came to characters in Mario franchise at the time, they were generally underdeveloped. Since Mario & Luigi is mainly based on a text-based story, AlphaDream approached Nintendo with the mindset of revamping characters such as Luigi. With Nintendo's approval and support, the characters and their depth were developed from scratch.

Nintendo revealed Mario & Luigi: Partners in Time (then called Mario & Luigi 2) at E3 2005, where a playable demonstration of the game was available. The demos consisted of three levels, each accompanied with a tutorial to guide the player. Each level had a different objective and represented the characters' abilities in the game, such as the use of the hammer. Between the game's unveiling at E3 and its release, Nintendo of America revealed details of the game relating to Partners in Time's plot and gameplay, as well as the fact that it would be compatible with the "Rumble Pak" feature. Partners in Time was first released in North America on November 28, 2005. Mario & Luigi: Bowser's Inside Story was revealed to be in development in October 2008 in Japan at Nintendo's Tokyo Press Event. At the time, the game was called Mario & Luigi RPG 3!!!. From there, it was announced in North America and Europe at the E3 2009 event and released in the fall of the same year.

With the introduction of three-dimensional graphics on the Nintendo 3DS, AlphaDream was given the opportunity to change their two-dimensional sprites to 3D ones with the updated hardware. However, Maekawa believed the company not only got very good at designing sprites due to limited graphical capability on prior consoles, but they also helped convey comical expressions, so they were kept. Instead, the backgrounds were fit to 3D. Akira Otani, a producer of the series, considered the animation to be the main reasoning for the extensive development process of Mario & Luigi: Dream Team. He mentioned how it takes up to six people to design the animations for the characters alone.