Since 1999, states have been required to submit to the Centers for Medicare & Medicaid Services (CMS) detailed enrollment and claims information for beneficiaries of Medicaid and the Children’s Health Insurance Program (CHIP). Known as the Medicaid Statistical Information System (MSIS) format, these state data submissions allowed national and cross-state analyses. As an initial national data system, MSIS data provided basic information on Medicaid and CHIP beneficiaries, their characteristics, enrollment patterns, service utilization, and costs of their care. However, as Medicaid and CHIP have evolved and become more complex programs, MSIS became inadequate as a national data system for these critical safety net programs. Recognizing this, CMS redesigned the national data system for Medicaid and CHIP, and, beginning in 2017, all states had to transition from MSIS to the Transformed MSIS, or T-MSIS, format. T-MSIS, the new national data system for Medicaid and CHIP, will be the source of information policymakers and others will use to understand who enrolls in these programs,what services are provided, the quality of the services received, and their costs. Mathematica is proud to support CMS as it rolls out T-MSIS and derivative data products. As a major provider of technical assistance to states, we worked with CMS to help each state and territory enter the T-MSIS production process. Since 2017, we have been providing several different forms of technical assistance to states to help them improve the quality of their T-MSIS submissions.