Dim declares a variable and its type before it is used. Typed variables are faster, catch mistakes, and get autocomplete for object members.

Code

Dim total As Double
Dim name As String
Dim i As Long

• Basic declarations

  Dim total As Double Dim name As String Dim i As Long → Three typed variables

  One per line is clearest.

• A classic trap

  Dim a, b As Long → a is Variant, only b is Long

  Each variable needs its own As.

• Object variable

  Dim ws As Worksheet Set ws = Worksheets("Data") → An object variable, assigned with Set

  Objects need Set.

• Declare at the top of the procedure so the reader sees every variable at once.
• Long for whole numbers, Double for decimals, String for text, Boolean for flags.
• Use Option Explicit so a missing Dim is an error.

• Dim a, b As Long declares a as Variant.
• A variable declared without As is a Variant, slower and loosely typed.