This article described many of the causes of monoarticular arthritis that are encountered in clinical practice. Although radiologists have relied on conventional radiography and bone scintigraphy, additional imaging methods have been introduced, such as MR imaging, CT scanning, and US. These methods improve visualization of intra-articular and periarticular soft tissue structures and are helpful in the guidance of arthrocentesis, drainage procedures, or percutaneous biopsies. Imaging findings always should be correlated with clinical abnormalities, and, when appropriate, joint fluid analysis.