With the increase over the past two decades in interest in critical thinking instruction, reliable and valid means for measuring critical thinking are necessary. The Minnesota Test of Critical Thinking was developed to measure critical thinking as defined by the American Philosophical Association in 1990. The present purpose was to explore the structure and reliability of the intervention and its instantiation of the critical thinking framework. Analyses of responses from 151 adult participants gave estimates of reliability varying from. 29 to. 78. A factor analysis of the test scores supported critical thinking conceptualized as a single, latent trait. With further refinement, the invention should become a valuable measure of general critical thinking skills of upper-division secondary and postsecondary students.