Most mass spectrometry-based proteomics cores routinely run QC materials to ensure reliable and informative results. Such QC samples are used to verify instrument performance, validate and benchmark methods within and across laboratories, and even comply with regulatory standards. Especially in multi-lab consortia, large sample sets, or long-term data collection, QC is imperative to combine data across instruments, laboratories, and time. Critically, QC samples allow separation of extrinsic measurement variance from intrinsic sample variability -something which is especially pertinent to laboratories serving a diverse group of users with varying experience levels. Historically, many mass spectrometry-proteomics laboratories have manufactured their own QC samples, for example by mixing selected compounds or digesting proteins from Escherichia coli or yeast whole-cell lysates. A number of mixtures suitable for QC purposes are now commercially available, including several developed in collaboration with Association of Biomolecular Resource Facilities (ABRF) Research Groups, aiding in material is a mixture of peptides (shown in cyan and red), a QC2 material is a whole cell or biofluids (shown as a green lined "cell" with its components) digest, while a QC3 material is mix of isotopically labeled peptides mixed into a more complex whole cell digest. Finally, a QC4 material is two or more samples of whole cell lysate or biofluid (labeled here as A and B) digest that have known mixtures of different species' proteomes (shown as green versus pink cells, at ratios of 2:1 or 1:2). Note this is only describing types of QC materials, while recommendations for implementing QC in experimental designs can be found in the Methods for Implementing QC section.