Since the mid-90s several studies have proven the existence of an Adult form of the Separation Anxiety Disorder (ASAD) not yet nosologically recognized by the international psychiatric classification systems (DSM and ICD). An increasing amount of evidence showed that the separation anxiety disorder may arise at any age, not always in continuation with the correspondent childhood disorder. So, a revision of the diagnostic criteria for this disorder is brought into question, as the onset is currently limited before 18 years of age. Different tools have been developed for the assessment of ASAD: 1) the Adult Separation Anxiety Structured Interview (ASA-SI), a semi-structured interview with items derived and adapted from the DSM-IV-TR childhood disorder; 2) the Adult Separation Anxiety-27 (ASA-27), a self-administered rating scale containing the same items of ASA-SI; 3) the Structured Clinical Interview for Separation Anxiety Symptoms (SCI-SAS), a structured interview including two specific forms for childhood and adulthood. However, according to available evidence, the separation anxiety may be a dimension with cross-nosographical presentation in nearly all the commonest mood and anxiety disorders; moreover, it is connected to greater personal dysfunction and lower responsiveness to treatment. Furthermore, a deeper comprehension of the psychobiological nature of separation anxiety should lead to newer and more effective therapeutic intervention. Literature is reviewed awaiting the publication of DSM-V.