To reliably write high performance code in any programming language, an application programmer must have some understanding of the performance characteristics of the language's core constructs. We call this understanding a performance model for the language. Some aspects of a performance model are fundamental to the programming language and are expected to be true for any plausible implementation of the language. Other aspects are less fundamental and merely represent design choices made in a particular version of the language's implementation. In this paper we present a basic performance model for the X10 programming language. We first describe some performance characteristics that we believe will be generally true of any implementation of the X10 2.2 language specification. We then discuss selected aspects of our implementations of X10 2.2 that have significant implications for the performance model. © 2011 ACM.