This section describes physics and instrumentation as directly applies to vascular ultrasonography and interpretation. Foundational principles are discussed while describing image acquisition, image optimization, visualization of signals with low contrast such as thrombus and masses, and spatial and temporal resolution. The more advanced topics of harmonic imaging, compound imaging, and ultrafast imaging are discussed as well as the underlying principles necessary to understand IMT measurements. The spectral Doppler section explains Doppler basics, angular error, angle correction, spectral broadening artifact, and the appropriate frequency for spectral Doppler data acquisition. Color Doppler includes a discussion on the topics of color scales (PRF), color wall filters, aliasing, appropriate color gain, color priority, power Doppler, microvascular flow detection, and the methodology for achieving maximum color sensitivity.