Option Explicit at the top of a module makes every variable require a Dim. Without it, a misspelled variable silently becomes a new empty one and the bug hides for hours.

Code

Option Explicit

• Catch a typo

  Option Explicit Sub Test() totl = 5 ' compile error: variable not defined End Sub → The typo is caught before the code runs

  This is the whole point.

• Turn it on for good

  Tools > Options > Require Variable Declaration → Option Explicit is added to every new module

  Set it once.

• Compile check

  Debug > Compile VBAProject → Every undeclared variable across the project is reported

  Find them all.

• Turn on Require Variable Declaration in Tools > Options so it is added to every module automatically.
• Add it to existing modules by hand; the option only affects new ones.
• Declaring types (As Long) also speeds code up.
• It must be the first line, before any procedure.

• Old code without declarations will not compile until every variable is declared.
• Forgetting it is the most common source of "it worked yesterday" bugs.