This article gives insight into the Dutch investigative practice of a specific type of undercover police strategy: infiltration. Infiltration is legally the most serious kind of undercover operation, which is mainly deployed against organized crime. Based on research covering all infiltration operations in the Netherlands during the 2000-2005 period, this article provides empirical insight into the use of this ‘last resort’ method of investigation. It looks into the numbers, course and results of infiltration operations but also into the practical outcomes of formal, regulatory requirements.