Cancer grade (histology) is a description of how cancer cells look under the microscope. It describes the extent to which cancer cells are similar in appearance and function to healthy cells of the same tissue type (also called differentiation).

Healthy cells grow and divide in an orderly fashion; cancer cells do not. Cancer cells are commonly "graded" into one of 4 groups—grades 1, 2, 3, and 4—depending on how rapidly they grow and divide. The cells of grade 1 cancers are more likely to resemble healthy cells. They are described as "well-differentiated" or "low-grade tumors," and they generally grow and spread slowly. The cells of grade 3 or grade 4 cancer do not look or behave like healthy cells. They are described as "poorly differentiated" or "undifferentiated high-grade tumors" and are generally the most aggressive in behavior. They spread and grow quickly.

The grading system is used to provide information about how fast a particular cancer will grow and spread. The systems used to grade tumors vary with each type of cancer. Knowing the grade of a cancer is helpful in planning appropriate treatment.