This digest discusses sources of error in testing, several approaches to estimating reliability, and several ways to increase test reliability. Reliability has been defined in different ways by different authors, but the best way to look at reliability may be the extent to which measurements resulting from a test are characteristics of those being measured. In a technical sense, the theoretical definition of reliability is the proportion of score variance that is caused by systematic variation in the population of test takers. Reliability is a joint characteristic of a test and examinee group, not just a characteristic of the test. There are three major sources of error: factors in the test itself, factors in the students taking the test, and scoring factors. It is impossible to calculate a reliability coefficient that conforms to the theoretical definition, but there are several statistics, or coefficients, commonly used to estimate the stability of a set of test scores for a group of examinees. The most commonly used statistics are: (1) test-retest reliability; (2) split-half reliability; (3) measures of internal consistency; and (4) alternate form reliability. When one considers how high reliability should be, the points to consider are the consequences of the test and whether the group used to compute the reported reliability is similar to the group of interest. If the consequences are high, then the internal consistency reliability should be high (at least above 0.90, and preferably above 0.95). Classroom tests seldom need to have exceptionally high reliability coefficients, since teachers have access to other sources of information about students. Developing tests with less error is better than simply documenting the amount of error. It is also commented that, especially in the elementary grades, a poor test result resulting from careless mistakes should not dramatically lower a student's grade for the semester. Teachers should ensure that the semester grade reflects what the student has achieved. (SLD)