Yellow Cyst on my kitty!

My cat has this little lump/cyst on his back. Should I be concerned? I have a vet appointment in a few days and am worried!

Any lump or bump, no matter how small, slight, location, etc, should always be checked by your vet, always.  That said, this could be as simple as a lipoma or fatty cyst, most of the time not worrisome.  If your vet is very experienced, he may be able to determine just upon exam.  It can also be aspirated and checked under a slide as a preliminary measure - if your vet isn't certain, he may want to do a biopsy to determine for certain (requires anesthesia).  As long as kitty is otherwise in excellent health and depending on his/her age, you should have blood work done prior to any anesthetic procedure.  Hopefully this is only a lipoma and nothing serious.

I am a human doctor, not a vet.  I am afraid that it is notoriously difficult to diagnose from a photograph and as  @cat-tech, writes,  I advise you to mention this little lump to your vet when you take up the appointment.  Please do not delay the appointment  more than a few days.

With best wishes,

Geoffrey