Published by Physical Review Letters on 29 Apr 2008.-- Final full-text of the paper available at: http://dx.doi.org/10.1103/PhysRevLett.100.173902.-- ©2008 The American Physical SocietyWe put forward a method that allows the experimental determination of the entire spatial mode spectrum of any arbitrary monochromatic wave field in a plane normal to its propagation direction. For coherent optical fields, our spatial spectrum analyzer can be implemented with a small number of benchmark refractive elements embedded in a single Mach-Zehnder interferometer. We detail an efficient setup for measuring in the Hermite-Gaussian mode basis. Our scheme should also be feasible in the context of atom optics for analyzing the spatial profiles of macroscopic matter waves.We acknowledge financial support from the Spanish Ministry of Science and Technology through Project No. FIS2005-01369, from Govern Balear (PROGECIB-5A), Juan de la Cierva and Ramon y Cajal Grant Programs, and Consolider Ingenio 2010 QIOT CSD2006-00019.Peer reviewed