A space followed by an underscore at the end of a line continues the statement on the next line, keeping long calls readable.

Code

MsgBox "Hello " & _
  "world"

• Split a string

  MsgBox "Hello " & _ "world" → One statement across two lines

  Long strings.

• Long method calls

  Range("A1:D100").Sort Key1:=Range("B1"), _ Order1:=xlDescending, Header:=xlYes → A long method call laid out one argument per line

  Readable calls.

• Long conditions

  If x > 0 And _ y > 0 Then → A condition split across lines

  Complex conditions.

• There is a limit of 24 continuations per statement.
• You cannot continue inside a string literal; close it, add &, then continue.
• Indent continued lines so the structure is visible.

• The underscore must be preceded by a space or it is a syntax error.
• A continuation at the very end of a procedure confuses the compiler.