Plant phenotyping technologies have been developing rapidly in the last decades. Among various phenotyping technologies, optical sensing such as RGB, multi-spectral and hyperspectral imaging have become popular and widely applied in different applications. In this chapter, these optical sensors are generally introduced first with a review of recently published literature in this area. With the objective of helping users to wisely select the right sensor for the right phenotyping application, this chapter also focuses on the comparison of advantages and disadvantages between the different sensor options. Particularly, the major bottleneck challenges around the optical sensors are discussed, including the distribution of stresses/nutrients/color across the canopy, the complexity of the canopy’s 3D structure, plant’s diurnal changes, and the impacts from the changing environmental conditions. Some potential solutions to these challenges are provided in the case studies, and the future technology development trend is also discussed.