As largest-scale computing systems currently progress to multi-petaflop systems and beyond, achieving their full potential requires increasing attention to the performance health of the systems overall, wherein degraded performance of a single subsystem such as a node, NIC, memory module, kernel process, etc. can effectively degrade the performance of an entire system running a large application. The current state of the art in identifying and remediating sources of performance loss is as much an art as a science, typically requiring labor-and expertise-intensive human resources operating in an ad-hoc and experience-based fashion. In this work we motivate the need for a new type of performance analysis tool, a Performance Health Monitor (PHM), that will efficiently pinpoint sources of lost performance on the largest systems and enable applications to experience a consistent performance environment from run to run. PHM is aimed at providing a global view of system performance in contrast to an aggregation of local views as well as to explain system performance issues and suggest corrective actions or pinpoint likely causes. A spectrum of usage modes will be supported ranging from quick, partial-system, userinitiated checks to comprehensive, full-system evaluations. Such evaluations may be archived as performance snapshots for comparing distinct systems, system subsets, or the same system pre-and post-upgrade.