Disclaimer: Here is a complete answer taking the last version of OS X (10.9 AKA Mavericks) into account. I am aware that everything I compiled in this answer is already present in the page, but having it clearly in one answer makes it a lot clearer.

First of all, with previous versions of OS X, Maven is installed by default. If Java is missing running you@host:~ $ java in a terminal will prompt you for the Java installation.

With Mac OS X 10.9 (Mavericks), Maven is not installed by default anymore. Different options are then possible:

• Using Homebrew:
  • you@host:~$ brew install maven will install latest Maven (3.5.2 on 02/01/2018)
  • you@host:~$ brew install maven30 will install Maven 3.0 if needed
• Using Macports: (I did not test this)
  • you@host:~$ sudo port install maven will install latest Maven (?)
  • or:
  • you@host:~$ sudo port install maven3 will Install Maven 3.0
  • you@host:~$ sudo port select --set maven maven3 selects that version of Maven
• Installing by hand:
  • Download Maven from its homepage
  • Follow the installation instructions:
    1. Extract the distribution archive, i.e.apache-maven-3.3.9-bin.tar.gz to the directory you wish to install Maven 3.3.9. The subdirectory apache-maven-3.3.9 will be created from the archive.
    2. Optional: Add the MAVEN_OPTS environment variable to specify JVM properties, e.g. export MAVEN_OPTS="-Xms256m -Xmx512m". This environment variable can be used to supply extra options to Maven.
    3. Make sure that JAVA_HOME is set to the location of your JDK, e.g. export JAVA_HOME=$(/usr/libexec/java_home -v 1.8) and that $JAVA_HOME/bin is in your PATH environment variable (although that might not be necessary with the latest Mac OS X versions and the Oracle JDK).
    4. Add extracted apache-maven-3.3.9/bin to your $PATH
    5. Run mvn --version to verify that it is correctly installed.