Monocular vision measurement extracts geometric information of real scenes using a single camera image, offering a lightweight and portable alternative to traditional multi-camera systems. This chapter introduces the basic principles of monocular measurement, including camera models, coordinate systems, and the use of homography to map image pixels to world coordinates on a planar surface. It then presents a complete measurement workflow: colour-based detection of calibration rulers and marker points, automatic and manual target selection, and distance and area computation. The chapter also analyses practical factors that influence accuracy, such as perspective distortion and the spatial relationship between calibration points and measurement targets.