At the request of the patient or required by a requisition, the physician may be asked to deliver medical certificates with determination of total work incapacity (TWI). This is an attestation of what he has observed and incurs his penal, civil and ethical responsability. The drafting of such certificates implies not only the doctor and the patient, but also the judicial authorities and the alleged offender or offenders. The impact of the determination of the TWI is important to know in order to write "good" certificates before justifying TWI.