This chapter is divided into two sections: the first lists the common and less common clinical features of atopic dermatitis (AD) with the aim of helping those less familiar with diagnosing atopic dermatitis in a clinical setting. The second part deals with diagnostic criteria for atopic dermatitis that may be used in research studies. The two sections are set apart deliberately since the requirements of a disease definition for making a clinical diagnosis in an individual may be somewhat different from making a group definition for research participants. A clinical sign such as thinning of the lateral eyebrows may be associated with AD, and possibly useful in the context of tipping a clinician to make a diagnosis of AD in a child with indeterminate skin inflammation. Yet it is too difficult to define reliably and too infrequent to be of use when examining groups of individuals, especially when it adds very little to the predictive ability of existing cardinal features such as flexural involvement and dry skin. Likewise, diagnostic criteria that may produce acceptable results for estimating the prevalence of AD in a country may not be suitable when trying to diagnose AD in an individual, since every set of diagnostic criteria will have limitations in sensitivity (proportion of true cases correctly identified) and specificity (proportion of non-cases correctly identified). All current diagnostic criteria will therefore misclassify some individuals who have AD as not having AD, and some who do not have AD as having AD. Such misclassification may be acceptable in research studies, providing the degree of misclassification is understood in relation to the objective of the study.