In this article we investigate how preservice elementary school (K-7) teachers understand the concept of prime numbers. We describe participants' understanding of primes and attempt to detect factors that influence their understanding. Representation of number properties serves as a lens for the analysis of participants' responses. We suggest that an obstacle to the conceptual understanding of primality of numbers is the lack of a "transparent" representation for a prime number. (Contains 1 figure, 1 table and 1 footnote.)