TRB’s Transportation Research Circular E-C129: Use of Inclinometers for Geotechnical Instrumentation on Transportation Projects explores the state of the practice and representative applications on the use of inclinometer systems for measuring ground deformation and performance of geotechnical design elements on transportation projects. The report examines inclinometer components and installation details, and includes information on planning inclinometer installation, acquiring and displaying data, and interpreting test results.