During the development process, the working group has identified the following design principles for the mzSpecLib format. First, it was decided that the standard should take the form of a common data model with multiple inter-convertible serializations that can serve different needs. For example, if human readability is desired, a text-based serialization would be the most suitable, while a JSON file would be best for softwareapplication data exchange. These two serializations are implemented at the present stage but more can be added in the future. This design choice is unique among PSI formats, but is necessary to reduce the barrier of entry and to address the sometimes conflicting demands of library builders and users. Second, terms defined in a controlled vocabulary (CV), PSI-MS , which is actively maintained by this PSI working group, are required for all metadata. This forces the producers and consumers of spectral libraries to use mutually agreed terminology with clear definitions to refer to the same concept. In addition, to ensure flexibility and extensibility, there was no formal restriction in the specification as to what terms may or may not be used in a spectral library, as long as they are defined in the CV. However, a table of commonly used terms is provided to guide the adoption of best practices in a separate "living" document maintained online as an addendum to the format. Third, the format should provide for a mechanism to minimize redundancy and file size by not encoding repetitive metadata in every spectrum. Meanwhile, any mechanism to do so must also ensure that the data integrity of the library can be maintained as libraries are split or merged, and as library entries are added or deleted. Finally, to set a manageable scope for the project, the inaugural version of the format will primarily support proteomics applications with basic small molecule support. Nevertheless, care has been taken that the overall design can be extended to other MS fields in the future (metabolomics, lipidomics, glycomics, etc).