This chapter evaluates ultrasound-based methods for assessing endometrial receptivity, a pivotal factor in embryo implantation success during assisted reproductive treatments. Key morphological parameters—including endometrial thickness, pattern (trilaminar vs. homogeneous), volumetric measurements, and contractile activity—are quantified to determine structural readiness for implantation. Complementary hemodynamic assessments analyze uterine artery pulsatility index (PI), resistance index (RI), and subendometrial vascularization indices, providing objective metrics of endometrial perfusion. Advanced imaging techniques such as 3D power Doppler and automated vascularity mapping further refine the characterization of endometrial-subendometrial blood flow distribution. The chapter also examines how ovarian stimulation protocols (e.g., GnRH agonist/antagonist regimens) may disrupt endometrial synchrony with embryonic development, highlighting strategies to optimize hormonal dosing and timing. In frozen embryo transfer cycles, progesterone-induced endometrial transformation is tracked through serial ultrasound to precisely identify the implantation window, minimizing asynchrony risks. By integrating structural, vascular, and functional data, ultrasound serves as a noninvasive tool for real-time endometrial evaluation. This multimodal approach supports personalized treatment adjustments—from cycle scheduling to embryo transfer timing—ultimately enhancing clinical pregnancy rates in patients with recurrent implantation failure or hormonally altered endometria.