Quality assurance of the analytical processes performed at the clinical microbiology laboratory is mandatory and should be carried out by using external and internal quality control activities. External quality assessment programs allow intercomparison within laboratories, detection of errors, and evaluation of the suitability of some reagents or diagnostic kits for the purpose for which they were designed; these activities are also useful for continuous education. The program launched 15 years ago by the Spanish Society of Infectious Diseases and Clinical Microbiology is based on sending typified materials along with a clinical and microbiological case related to these control materials. The spectrum of the samples is broad, including bacteriology (monthly and three-monthly), serology, mycology, parasitology, mycobacteria, virology, and molecular microbiology. After receiving the results from the participants, the program organization delivers an individual certificate comparing the results with those of a reference laboratory. Additionally, a report is generated by analyzing all the results sent by the participants; laboratories are also sent review articles on the subject of each assessment as a tool for continuous education in clinical microbiology. In this article, the most relevant conclusions and lessons from the 2005 assessments are presented.