I disagree with the article linked above. Food causes acne and there are good reasons for it. The short answer is that hormone irregularities and genetic tendencies are the root causes of acne. These trickle down to the more proximate causes of increased sebum (oil) production and improper skin cell shedding which clogs pores.
If you still have acne past your teens it is likely due to your overall health. Puberty causes insulin resistance which makes hormone irregularities occur more easily. Once through puberty, insulin resistance goes down but if you eat a lot of high-glycemic index foods your hormones will still be out of whack. You can read more at my what causes acne page.