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A module in modern Fortran is a namespace + interface unit. Use it to group related procedures, types, and constants.

module math_utils
    implicit none
    private
    public :: square, sum_array, PI

    real, parameter :: PI = 3.14159265

contains
    pure function square(n) result(s)
        integer, intent(in) :: n
        integer :: s
        s = n * n
    end function

    pure function sum_array(a) result(s)
        integer, intent(in) :: a(:)
        integer :: s
        s = sum(a)
    end function
end module

private / public — control what's exported. By default everything in a module is public.

use brings module contents into scope:

program main
    use math_utils
    implicit none
    print *, square(7)
    print *, PI
end program

Selective import with only:

use math_utils, only: square

Renaming:

use math_utils, only: sq => square
print *, sq(7)

Why modules matter:

• Type-checked interfaces — compile-time errors instead of segfaults
• Encapsulation
• Compiler can optimize aggressively across module boundaries
• Replaces older external and common blocks

Modules are stored in .mod files alongside object files. Compile order matters — modules must be compiled before code that uses them.