Do you need to sand down old paint before painting with gloss?

Do I need to create a 'key' to the old gloss by sanding it down?Do you need to sand down old paint before painting with gloss?
Just enough to rough up the surface before applying the new. Something to look for, just make the surface appear ';milky'; and that should be sufficient.Do you need to sand down old paint before painting with gloss?
Yes if you want a good finish, preparation is all.
Allways sand any surface before painting .