Muscle disease can affect skeletal (striated), cardiac and, more rarely, smooth muscle. It can be primary or secondary to other systemic disorders but can be difficult to recognize clinically owing to its non-specific signs. Even when muscle disease is supected, the appropriate diagnostic work-up is often poorly understood and test results can be misinterpreted. This chapter discusses clinical sign sof muscle disorders, laboratory evaluation of muscle disorders and tests used in the investigation of skeletal (striated) muscle disorders. Case examples are included.