The majority of States set a legal age limit, between 10 and 14 years old, at which a child is presumed competent to testify. Testimonial capacity is based on four general factors: understanding of the difference between truth and falsehood, mental capacity to observe and receive accurate impressions of the occurrence, sufficient memory for an independent recollection of the event, and ability to communicate this memory. Forensic assessment of the child witness aims at determining if there is mental disease or defect and whether the functions relevant to being a witness are directly impaired. Sources of data for assessment include a mental status examination, clinical interview, caretaker's report, and relevant past history. Psychiatric assessment should consider the child's level of function, intactness of memory, willingness and ability to testify, anxiety, and understanding of court procedure and ability to tolerate it. The competency report should provide information on the reason for referral, sources of data, description of child's functioning, relevant past history, clinical observations and impression, and an opinion. Case examples highlight major clinical issues in competency and credibility examinations. 2 tables and 29 references.