Monster, P.I. is an app-based, gamified, computer-adaptive diagnostic assessment developed by a team of experts with a federal* research grant. The app measures written language skills (knowledge of morphology, vocabulary, and syntax) of 5th to 8th graders and provides teachers with interpretable score reports to drive instruction that improves vocabulary, reading, and writing abilities. Linked to common core language and reading standards, the app features a mischievous monster who is wreaking havoc! Students have to solve word puzzles to earn clues to identify and catch the monster, saving the city. Monster, P.I. is supported on browsers and native mobile platforms. Locally saved scores are synced remotely, allowing for use in offline and low-connectivity classroom environments. The iPad-based system enables students to spell, choose answers, and hear audio pronunciation of words. Teachers import rosters and download results in spreadsheet format.