For compilation of GRASS source code, see Compile and Install.
GRASS requires a workstation running some flavor of UNIX conforming to POSIX standards like Linux, Mac OS X, Solaris, IRIX, or BSD. It is also possible to run GRASS in MS Windows by using UNIX translation software such as Cygwin or MinGW. Ideally, you should have at least 500 Mb for data and 128 Mb RAM. The source code package needs around 30 MB uncompressed. The resulting binaries may need between 20 MB and 180 MB depending on your platform. During a full compilation you may need temporarily up to 150MB including the source code.
The Software Download Section of the main GRASS web site contains the latest binaries and source code for all supported platforms. That site also has general directions for installing GRASS manually. However, installation is slightly different on each operating system. Here you can find user-contributed pointers for installing GRASS on specific platforms. In particular, many operating systems have package management utilities that can greatly simplify GRASS installation.
A binary version of GRASS is available from the apt repository. As root type:
apt-get install grass grass-doc
This is the easiest way to install GRASS on Debian. If you choose to install a binary version manually from the main web site, be sure to follow the instructions for making symlinks found as a note to the [6.1 weekly snapshot] release.
There is also the wonderfull DebianGIS project which has a more recent GRASS version with its related packages.
To install from there follow the instructions there
Compiling GRASS from source on Debian
Packages needed for (nearly) complete GRASS installation from source code:
flex, bison, libreadline4-dev || libreadline5-dev, libncurses5-dev, lesstif2-dev, debhelper (>= 4.0.2), dpatch, libtiff4-dev, tcl8.4-dev, tk8.4-dev, fftw-dev, xlibmesa-gl-dev, libfreetype6-dev, autoconf2.13, autotools-dev, libgdal1-1.3.1-dev, proj (>= 4.4.7), libjpeg62-dev, libpng12-dev, postgresql-dev, unixodbc-dev, doxygen, fakeroot
Intevation provides RPM-packages for stable releases prepared to install on Fedora systems.
GDF Hannover provides RPM-packages for stable releases prepared to install on Mandriva systems.
You can download them here: 
GDF Hannover provides RPM-packages for stable releases prepared to install on SUSE-systems.
You can download them here: 
GRASS Binaries are available from apt/synaptic. From a terminal type:
sudo apt-get install grass grass-doc
or alternatively, search for and install these packages from Synaptic. This is the easy way to get GRASS on your system. Even if you choose to install binaries from another source, you may want to install this version just so that all (most) dependencies are installed as painlessly as possible.
To get the latest version of GRASS on Ubuntu, compile the code from source. See the Compile and Install section for a shell script that makes this easy.
Mac OS X
Precompiled Mac OSX packages can be found here.
- Precompiled winGRASS/Cygwin 6.1 packages are provided here (requires Cygwin unix emulator, see there).
- Precompiled native winGRASS packages with QGIS integrated are provided here (no Cygwin needed).