Objective:To present the Mobile Toolbox (MTB), comprised of an expandable library of cognitive and other tests, including adapted versions of NIH Toolbox® measures. The MTB provides a complete research platform for app creation, study management, data collection, and data management. We will describe the MTB project and MTB research platform and demonstrate examples of assessments.Participants and Methods:MTB is the product of an NIH-funded, multi-institutional effort involving Northwestern University, Sage Bionetworks, Penn State, University of California San Francisco, University of California San Diego, Emory University, and Washington University. The MTB assessment library is a dynamic repository built upon Sage Bionetworks mobile health platform. All MTB measures are created or adapted for a mobile interface using iOS and Android smartphones. Guided by the principles of open science, many components are open source to allow researchers and developers to integrate externally developed tests, including supplemental scales (e.g., passively collected contextual factors) assessing variables such as mood and fatigue that might influence cognitive test performance.Results:The current MTB library includes eight core cognitive tests based on well-established neuropsychological measures: two language tasks (Spelling and Word Meaning), two executive functioning tasks (Arrow Matching and Shape-Color Sorting), an associative memory task (Faces and Names), an episodic memory task (Arranging Pictures), a working memory task (Sequences) and a processing speed task (Numbers and Symbols). Additional cognitive assessments from other popular test libraries including the International Cognitive Ability Resource (ICAR), Cognitive Neuroscience Test Reliability and Clinical Applications for Schizophrenia (CNTRACS) and Test My Brain are currently being implemented, as are non-cognitive measures from the NIH Toolbox Emotion Battery and the Patient-Reported Outcomes Measurement Information System (PROMIS). The MTB library includes measures suitable for use in research studies incorporating point-in-time and burst designs as well as ecological momentary assessment (EMA).Conclusions:The MTB was created to address many of the scientific, practical, and technical challenges to cognitive assessment by capitalizing on advances in technology measurement and cognitive research. Initial psychometric evaluation of measures has been performed, and additional clinical validation is underway in studies with persons at risk for cognitive impairment or Alzheimer’s disease (AD), diagnosed with mild cognitive impairment (MCI) or AD, Parkinson’s disease, and HIV-associated Neurocognitive Disorders. Calculation of norms and reliable change indicators is in progress. The MTB is currently available to beta testers with public release planned for Summer, 2023. Clinical researchers will be able to use the MTB system to design smartphone-based test batteries, deploy and manage mobile data collection in their research studies, and aggregate and analyze results in the context of large-scale norming data.