To make effective use of parallel computing environments, users have come to expect a broad set of tools that augment parallel programming and execution infrastructure with capabilities such as performance evaluation, debugging, runtime program control, and program interaction. The rich history of parallel tool research and development reflects both fundamental issues in concurrent processing and a progressive evolution of tool implementations, targeting current and future parallel computing goals. The state of tools for parallel computing is discussed from a perspective of performance evaluation. Many of the challenges that arise in parallel performance tools are common to other tool areas. I look at four such challenges: modeling, observability, diagnosis, and perturbation. The need for tools will always be present in parallel and distributed systems, but the emphasis on tool support may change. The discussion given is intentionally high-level, so as not to exclude important tool projects by citing others. Rather, I attempt to present viewpoints on the challenges that I think would be of concern in any performance tool design.