Storing Part Of An Express To A Variable

I know this can be done, because I saw it and tried it once. But did not use it at the time and consequently forgot how.

I am storing an expression to a variable @Var Varchar(50)

@Var = ('aa','bb','cc')

I want to:

Select * from Table WHERE Field IN @Var

Substituting my variable for the " ('aa',bb','cc') which I can build dynamically...thus the use.

Purpose:
I have an aggregate query that currently has a CASE statement based on the contents of ('aa',bb','cc'). However, those contents can change. The aggregate will not allow the subquery (SELECT Field FROM Table).

What I remember looked something like:

SELECT * FROM Table WHERE field in ' & @Var &'.