The seminal vesicle (SV) and vas deferens (VD) are ancillary but essential urogenital organs. Understanding their embryologic features and anatomy can be helpful in evaluating various disorders of these organs. Recently, cross-sectional imaging modalities, including ultrasonography, computed tomography, and magnetic resonance (MR) imaging, have been increasingly used for evaluation of the SV and VD. The development of these organs is closely related to that of urinary organs, including the kidneys and ureters. Frequently, unilateral SV agenesis is associated with renal agenesis, and bilateral SV or VD agenesis is associated with mutations of the cystic fibrosis gene. Congenital SV cysts are commonly associated with ipsilateral renal agenesis or dysgenesis. These congenital anomalies can be well evaluated with MR imaging. Inflammation, post-radiation therapy changes, and amyloidosis of the SV appear as diffuse wall thickening and may mimic tumor invasion by prostate cancer. Primary neoplasms involving the SV and VD are extremely rare, whereas secondary neoplasms are much more common. Carcinoma from the prostate, bladder, or rectum can directly invade the SV and VD. Typical MR imaging findings of such invasion include a low-signal-intensity mass on T2-weighted images or soft-tissue thickening in the SV or VD along with loss of normal architecture.