Lazarus is a cross-platform, integrated development environment (IDE) for rapid application development (RAD) using the Free Pascal compiler. Its goal is to provide an easy-to-use development environment for developing with the Object Pascal language, which is as close as possible to Delphi. It is free and open-source software with different parts released under different software licenses.

Lazarus is often used to create native-code console and graphical user interface (GUI) applications for desktop computers, mobile devices, web applications, web services, visual components, and function libraries for several different operating system platforms, including macOS, Linux, and Windows.^[1]

A project created by using Lazarus on one platform can be compiled on any other one which Free Pascal compiler supports. For desktop applications, one source code can target macOS, Linux, and Windows, with little or no modification. For example, the Lazarus IDE is created from one code base and available on all major platforms including Raspberry Pi.

Lazarus provides a What You See Is What You Get (WYSIWYG) development environment for creating rich user interfaces, application logic, and other supporting code artifacts, similar to Delphi. Along with project management features, the Lazarus IDE also provides:

• A visual windows layout designer
• GUI widgets or visual components such as edit boxes, buttons, dialogs, menus, etc.
• Non-visual components for common behaviors such as persistence of application settings
• Data-connectivity components for MySQL, PostgreSQL, FireBird, Oracle, SQLite, Sybase, and others
• Data-aware widget set that allows the developer to see data in visual components in the designer to assist with development
• Interactive debugger
• Code completion
• Code templates
• Syntax highlighting
• Context-sensitive help
• Text resource manager for internationalization
• Automatic code formatting
• Extensibility via custom components

Lazarus uses Free Pascal as its back-end compiler. As Free Pascal supports cross-compiling, Lazarus applications can be cross-compiled from Windows, Linux, or macOS to any of the supported Free Pascal compilation targets. Applications for embedded devices (smartphones, PDAs, routers, game consoles) can be cross-compiled from any desktop platform.

Lazarus provides a cross-platform application framework called the Lazarus Component Library (LCL), which provides a single, unified interface for programmers, with different platform-specific implementations. Using LCL, it is possible to create applications in a write once, compile anywhere manner, unless system-dependent features are used explicitly. LCL was originally modeled after the Visual Component Library (VCL) in Delphi 6, but is not restricted to Windows. This is done by separating the definition of common widget classes and their widgetset-specific implementation. Each widget set is supported by providing an interface which interacts directly with the set.^[2]

Developers can install packages that allow Lazarus to support several database management systems (DBMSes). Programs can interact with a DBMS through code or by components dropped on a form.

The following DBMSes are supported out of the box:

• dBase and FoxPro,
• InterBase and Firebird^[3]
• Microsoft SQL Server and Sybase ASE^[3]
• MySQL and MariaDB^[3]
• Open Database Connectivity (ODBC) databases^[3]
• Oracle Database^[3]
• PostgreSQL^[3]
• SQLite^[3]

Lazarus resembles Delphi in many ways. It supports Component Object Model (COM) since version 2.2.0, and offers most of Delphi's networking functions. However, there are limits to the performance and feature set.

Lazarus does not support the following, which Delphi does:

• Datasnap (uses Embarcadero proprietary enterprise functions, not a publicly documented system)
• Dynamically loadable packages
• .NET libraries (.NET code, however, can invoke Object Pascal code or anything other machine-native library via Platform Invocation Services.^[4])
• Extensive Microsoft Office connectivity (Lazarus can only open Microsoft Excel with a simple table filled out.)^{[citation needed]}

LCL is not fully compatible with VCL. This makes the extensive repository of available VCL widgets inaccessible without conversion. The conversion effort mostly involves some editing, although there are a few fundamental differences. When porting, missing units in the libraries are a considerably bigger problem than incompatibilities between LCL and VCL. Components for Delphi can be converted to work in Lazarus. This can be complex, though less so than for Lazarus versions older than 0.9.30, based on FP 2.4.x.

On Windows, the default size of an executable file is larger than the Delphi 6 or 7 equivalent, as Lazarus stores debug information within the executable, rather than as separate files. Starting with version 0.9.30, Lazarus supports external debug symbols via compiler options. Thus, program file sizes can be significantly reduced. Alternatively, debug info can be stripped from EXEs (e.g. using a port of the UNIX strip command).

Like Free Pascal, Lazarus is free software. Different portions are distributed under different free software licenses, including GPL, LGPL, MPL, and a modified version of LGPL.^[5] LCL, which is statically built into the produced executables, is licensed under a modified version of the LGPL, granting extra permissions to allow it to be statically built into the produced software, including proprietary ones.

Installing a design time package is equivalent to linking to the IDE, so that distributing the Lazarus IDE with a GPL-incompatible design-time package such as the JEDI packages licensed under the Mozilla Public License, pre-installed would cause a license violation. This does not prohibit proprietary packages from being developed with Lazarus.

The first attempt to develop an IDE for Free Pascal dates back to 1998, under the "Megido" project. After the project failed, some of its developers started a new project based on a more flexible foundation. The name "Lazarus" alludes to the revival of the Megido concept. It is inspired by Lazarus of Bethany, who, according to the Gospel of John, was restored to life by Jesus four days after his death.

The first preliminary LCL version was ready for release in 2001. In 2003, the first beta version of Lazarus (0.9.0.3) was hosted at SourceForge. Lazarus version 1.0 was released in 2012. A significantly enhanced Lazarus 1.2 with was released in 2014. More than four million downloads had been made from SourceForge as of March 2014.

• ASuite is a free open-source application launcher for Windows. From 2.1 Alpha 1, it's fully written in Lazarus/FPC.
• Beyond Compare is a data comparison utility for Windows, macOS, and Linux. The macOS and Linux versions are compiled using Lazarus/FPC.
• Cartes du Ciel is a free planetarium program for Linux, macOS and Windows. The software maps out and labels most of the constellations, planets, and objects you can see with a telescope. It is fully written in Lazarus/FPC and released under GPL.
• C-evo is an open source turn-based strategy game that has been ported from Delphi to Lazarus.^[11]
• Cheat Engine is an open-source memory scanner/hex editor/debugger. It is useful for cheating in computer games. Since version 6.0 it is compiled with Lazarus/FPC.
• EPANET, a software package for modelling water-distribution systems.
• HNSKY, Hallo Northern Sky is a free planetarium program for Windows and Linux. Since version 3.4.0 written and compiled with Lazarus/FPC.
• MyNotex is a free software for Linux useful to take and manage textual notes.
• PeaZip is an open-source archiver, made with Lazarus/FPC.
• TorChat is moving away from Python and is being rewritten in Lazarus + Free Pascal.
• Total Commander 64-bit version.^[12]
• Double Commander is a cross-platform open-source file manager with two panels side by side. It is inspired by Total Commander, plus some new ideas.^[13]
• SimThyr is a continuous simulation program for thyroid homeostasis.

• GLScene is a free OpenGL-based library that provides visual components and objects allowing description and rendering of 3D scenes.
• OpenWire is an open-source library that allows pin type properties to make connections between LCL components similar to LabVIEW or Agilent VEE.

• Comparison of integrated development environments#Pascal, Object Pascal
• Dev-Pascal
• Delphi (software)
• Free Pascal Compiler
• Object Pascal