The previous chapter dealt with point estimation and related problems in linear mixed models. In this section, we consider a different type of inference, namely, tests in linear mixed models. Section 2.1.1 discusses statistical tests in Gaussian mixed models. As shown, exact F-tests can often be derived under Gaussian ANOVA models. Furthermore, in some special cases, optimal tests such as uniformly most powerful unbiased (UMPU) tests exist and coincide with the exact F-tests. Section 2.1.2 considers tests in non-Gaussian linear mixed models. In such cases, exact/optimal tests typically do not exist. Therefore, statistical tests are usually developed based on asymptotic theory. 2.1.1 Tests in Gaussian Mixed Models 2.1.1.1 Exact Tests For ANOVA models, exact F-tests can often be derived using the following method. The original idea was due to Wald (1947). Consider the mixed ANOVA model (1.1) and (1.2). Suppose that one wishes to test the hypothesis H 0 : σ 2 1 = 0. Note that the model can be written as