The grant review essentially concerns scientific assessment, a critical topic in grantology and a specific area in the science of science. Considering the intuitive knowledge outlined at the start of the chapter, the chapter is intended to focus on the current scientific understanding of grant review, especially on key concepts and challenging issues (e.g. reviewers, review criteria, review processes, review outcomes) to help in preparing the writing of the grant proposal and achieve success in grant applications. While there are different funding agencies and diverse grant review procedures, this chapter does not exhaust all the grant review approaches. Instead, it focuses on major important and challenging issues in grant review. It focuses on the complexity and diversity of grant review.