Book excerpt: Introduction -- Comparison of Photon and Thermal Detectors Performance -- GaAs/AIGaAs Based Quantum Well Intra-red Photodetector Focal Plane Arrays -- GaInAs(P) Based Qwips on GaAs, InP and Si Substrates for Focal Plane Arrays -- InAs/(Galn)Sb Superlattices: A Promising Material System for Infra-red Detection -- GaSb/InAs Superlattices for Infra-red FPAs -- MCT Properties, Growth Methods and Characterization -- HgCdTe 2D Arrays -- Technology and Performance Limits -- Status of HgCdTe MBE Technology -- Silicon Infra-red Focal Plane Arrays -- PolySiGe Uncooled Microbolometers for Thermal Infra-red Detection -- Infra-red Silicon/Germanium Detectors -- Fundamentals of Spin Filtering in Ferromagnetic Metals with Application to Spin Sensors.