Several authors have stressed the usefulness of ethology and its tools of systematic behavioural assessment in psychiatric practice and research (e.g. Grant 1972; Troisi 1999; Phillipot e.a. 2003). However, as Troisi (1999) puts it: 'In psychiatric research, while almost all the physiological data about a patient are measured to at least one decimal point on parametric scales, the behaviour of the patient is given only ordinal ratings' and 'behavioural assessments are still the Achilles heel of biological psychiatric research'. This series of papers that presents findings from both child and adolescent psychiatry and from adult psychiatry, attempts to meet these critical remarks. In the papers behavioural observation data of patients with schizophrenia, with autism, and with depression will be linked to different theoretical approaches of psychiatric illness (i.e. Theory of Mind, Interpersonal Theories of Depression, and Attachment Theory). The results underscore the usefulness of systematic and detailed behavioural observations in psychiatric research. The language will be English.