How simulate a logical 'OR' and 'AND in T-SQL?

I know we have an IF and ELSE in Sybase SQL but I'm pretty sure there's no logical OR or AND, am I right?  Is there a way to simulate these?  I guess I can simulate "IF condition1 and condition2 then dothis" with:

IF condition1

BEGIN

     IF condition2

     BEGIN

          dothis

     END

END

But I'm not sure how to simulate an "or".  Any help would be appreciated.