Sometimes beside and besides are confused especially with writing.
The word beside is a preposition. It means close  to or next to.
   Come  and sit beside me.
 He  lives beside a Turkish take away.
As a preposition, the word besides means apart  from or in addition to.
   Besides  Sarah, who else went camping for the weekend?
 (  here 'besides' means 'apart from')
I  wasn't too keen to go to the concert. Besides, I was too busy.
   (here  'besides' means 'in addition to', 'furthermore')
Lesson by Tristan, teacher at EC Malta English school
Now complete the following with beside or besides: