REM This is a remark...
' This is also a remark...

IF a = 0 THEN REM (REM follows syntax rules)
IF a = 0 THEN '(apostrophe doesn't follow syntax rules, so use END IF after this)
END IF

'Metacommands such as $DYNAMIC and $INCLUDE use the REM (or apostrophe).
REM $STATIC 'arrays cannot be resized once dimensioned.
REM $DYNAMIC 'enables resizing of array dimensions with REDIM.
REM $INCLUDE: 'loads a reference file or library.