Long-term observation is a dual-purpose measure in combating organised crime, as it serves both preventive and repressive purposes. The main problem that can be raised in Portugal is the admissibility of such surveillance in criminal proceedings, particularly the use of the evidence thereby obtained. The execution of these measures restricts the right to privacy, the right to informational self-determination and, when technical means that record voice and image are used, the right to one’s image. Although long-term observation is not expressly regulated in Portuguese criminal procedural law, surveillance reports and witness testimony of the police officers who carried it out are widely admitted by the Portuguese courts. However, given the severe restriction of fundamental rights and the absence of legal provision, it must be concluded that evidence resulting from such surveillance is null and may not be used. For long-term observation to be legitimately admitted, the Portuguese legislator must establish a legal basis defining the conditions under which it may be used, in line with other covert methods of investigation.