The use of hadron beams, especially proton beams, in cancer radiotherapy has 
expanded rapidly in the past two decades. To fully realize the advantages of 
hadron therapy over traditional x-ray and gamma-ray therapy requires accurate 
positioning of the Bragg peak throughout the tumor being treated. A half century 
ago, suggestions had already been made to use protons themselves to develop 
images of tumors and surrounding tissue, to be used for treatment planning. The 
recent global expansion of hadron therapy, coupled with modern advances in 
computation and particle detection, has led several collaborations around the 
world to develop prototype detector systems and associated reconstruction codes 
for proton computed tomography (pCT), as well as more simple proton radiography, 
with the ultimate intent to use such systems in clinical treatment planning and 
verification. Recent imaging results of phantoms in hospital proton beams are 
encouraging, but many technical and programmatic challenges remain to be 
overcome before pCT scanners will be introduced into clinics. This review 
introduces hadron therapy and the perceived advantages of pCT and proton 
radiography for treatment planning, reviews its historical development, and 
discusses the physics related to proton imaging, the associated experimental and 
computation issues, the technologies used to attack the problem, contemporary 
efforts in detector and computational development, and the current status and 
outlook.