The state-of-the-art modal identification for civil structures is limited to the estimation mode shapes with a low spatial resolution. Modal coordinates are identified only at the location of sensors, which are fixed at particular locations on the structure. Increasing the number of sensors in the structure is an alternative to increase the spatial resolution. Unfortunately, this increases the cost of the instrumentation and the data to be transmitted and processed. Another common alternative is to use numerical algorithms to expand the modal coordinates to nonmeasured degrees of freedom. However, mode shape expansion techniques could introduce errors in the identified modes. This paper presents the formulation, evaluation, and validation of a methodology for the ambient vibration-based modal identification using mobile sensors (MIMS). The methodology uses two sensors, one mobile and one stationary sensor, to identify spatially dense modes of vibration. The methodology is experimentally verified by using a uniform simply supported steel beam. Results show that MIMS is able to identify dense mode shapes using a fraction of sensors and data when compared with traditional modal identification techniques. (C) 2016 American Society of Civil Engineers.