Quality control (QC) is a subset of quality systems, which for mass spectrometry cores includes everything from sample preparation to data acquisition and initial analysis. It is useful, arguably essential, to isolate and evaluate instrument performance before, during, and after a series of sample measurements (known as a run), independently of the experimental samples. This is often referred to as system suitability testing (SST). Likewise, QC materials of varying complexity and relevance can be processed and run parallel to experimental samples to provide confidence in the total workflow from samples to results. Providing SST and QC performance along with experimental results is essential, especially when results are negative or perceived as suboptimal.