Observation is considered a reliable method for the recognition of behavioural, communicative, cognitive, sensory and motor difficulties. However, the assessment of children with pervasive developmental disorders (PDD) should not be confined to the application of one method (Kroustalakis 1994, Polychronopoulou 1995, Vosniadou, 1995). This article aims to present the method of systematic observation (value, aim, techniques and the role of observer), as well as to propose other techniques of assessment and diagnosis for children with PDD. The conclusion is that since PDD characteristics change during a child’s development and they vary among individuals, since diagnostic criteria and tools have not yet been commonly decided among professionals, causes of PDD are still under investigation and treatments have not been defined, systematic observation can contribute to the assessment of children with PDD and make the environment functional to child’s needs.