The Physics of Automatic Target Recognition is part of a series focusing on Advanced Sciences and Technologies for Security Applications. This book will address the fundamental physical bases of sensing, and information extraction in the state-of-the art automatic target recognition field. We will explore both passive and active multispectral sensing, polarimetric diversity, complex signature exploitation, sensor and processing adaptation, transformation of electromagnetic and acoustic waves in their interactions with targets, background clutter, transmission media, and sensing elements. The general inverse scattering, and advanced signal processing techniques and scientific evaluation methodologies being used in this multi disciplinary field will be part of this exposition. The issues of modeling of target signatures in various spectral modalities, LADAR, IR, SAR, high resolution radar, acoustic, seismic, visible, hyperspectral, in diverse geometric aspects will be addressed. The problem of clutter modeling from a physics point of view will be also explored. The methods for signal processing and classification will cover concepts such as sensor adaptive and artificial neural networks, time reversal filter. The issue of invariants of sensor and transmission transformations (geometrical, spectral and polarimetric invariants) will be explored . These invariants are crucial in the development of low latency and computationally manageable ATR systems. The book will address the issue of transformations that a signal goes through in its interactions with targets and background, in its passage through a medium and its final reception by a sensor. It is only through understanding these transformations that one can hope in addressing the inverse problem: the identification of originating sources of a signal (target recognition). Advanced Sciences and Technologies for Security Applications focuses on research monographs in the areas of -Recognition and identification (including optical imaging, biometrics, authentication, verification, and smart surveillance systems) -Biological and chemical threat detection (including biosensors, aerosols, materials detection and forensics) -Secure information systems (including encryption, and optical and photonic systems). The series is intended to give an overview at the highest research level at the frontier of research in the physical sciences.