ClinicalTrials.gov is a database designed to help clinical researchers make their research publicly available. The clinical trials registered on the database each include a brief summary, which is meant to be a short description that the public can easily understand. In September 2022, ClinicalTrials.gov published a "Plain Language Checklist for Lay Brief Summaries" on their website, which identifies plain language best practices intended to help investigators craft summaries that can be readily understood by the public. This thesis assesses the impact of the checklist on the language use in the brief summaries in the year following the checklist's publication. The analysis examines 62 brief summaries for Phase III and IV clinical trials posted on ClinicalTrials.gov between September 26, 2022, and September 26, 2023. It focuses particularly on summaries associated with rheumatoid arthritis, knee replacement, and conjunctivitis to gauge how well they complied with 4 of the 19 criteria on the Plain Language Checklist: keeping sentences and paragraphs short, aiming for a 6th to 8th grade reading level, writing out acronyms on the first use, and providing both percentages and natural frequencies. It also examines rhetorical moves made in the summaries to address the use of jargon, key term definitions, headings, formatted lists, direct research questions, descriptions of study type, sentence fragments, and the placement of the purpose statement to see how these moves affected the plain language. Although the summaries tended to comply with the paragraph length guidelines, they did not comply with the sentence length, reading level, or acronym guidelines. The variation in compliance could be attributed to researchers' lack of awareness of the guidelines, lack of time to devote to creating brief summaries, or being too immersed in the field to imagine the needs of a lay audience. It could also be attributed to the National Institute of Health not enforcing the guidelines or to researchers not viewing the guidelines as being relevant.