I'm always a bit wary of buying clothing labeled S/M/L.  Using the Amazon sizing guide, I bought this costume in a Medium, which would fit an infant 6-9 or 9-12 months.  9 months would be perfect, so I figured I was ok.  The costume I received was actually labeled 12-18 months, and is HUGE on my son.  The costume itself is great, but beware of the size when you purchase.  You may not be getting what you think you're getting.