A Fortran program consists of a main program augmented by program units (subroutines, functions, and modules). The main program is a program unit itself. Each program unit has a declaration section, where variables, etc. are specified, followed by an execution section with executable (at run-time) statements. Program units end with an end statement.

A simple "hello world" program looks like this:

program hello_world
  write (*,*) 'Hello World!'
end program hello_world

Here we are using the write command to print to the screen. The first argument of this command specifies where to print and what format to use. For this simple statement we use (*,*). The first star specifies printing to standard out. The format (the second star) is left to Fortran. More on I/O follows in a later section.

Alphanumeric characters (letters, underscore, and numerals) are used in Fortran tokens (keywords) and variables. Special characters may appear in a variety of contexts. Blanks may be used freely to improve layout:

/   ( )   ,   =   =>   :   ::   ;   %

Blanks are ignored except when they are in literal strings. This is strictly true for the free format. This means that blanks may be inserted into keywords. Also variable names may have blanks; the variable "sum" may be written as "s u m" . Neither practice is recommended. Fixed format is stricter and does not allow blanks in keywords or variable names.

There is no distinction between upper and lower case, except in literal strings. Variables start with a character and can include numerals and the underscore. In Fortran 90 and 95, the length of a variable (and function and subroutine) names is at most 31 characters.

Fortran supports 5 basic types of variables: real, integer, logical (Boolean), complex (complex variables), and character (strings). Literal constants of these types are: