The preanalytical phase includes a set of processes that are difficult to define because they take place in different places and at different times. Classically, the preanalytical phase included all processes from the time a laboratory request is made by a physician until the sample is ready for testing. While this definition is very informative, errors that occur at this stage often become apparent later in the analytical and post-analytical phases. For example, interference effects for a given sample can be detected during the laboratory analysis or during clinical interpretation. Therefore, current recommendations call for the laboratory error to be defined as the defect occurring at any point in the cycle, from request to interpretation of results by a clinician. The main processes that should be taken into account in the study of the preanalytical phase are: test selection; patient preparation; collection, transport, handling and preservation of the sample; and interferences. The study of the characteristics of individual patients and the biological variation for each laboratory test belong to this phase, too. It has been documented that laboratory errors have significantly decreased in the last four decades, especially those that occur during the analytical phase; furthermore the scientific evidence reveals that most laboratory errors occur in the preanalytical phase. The large majority of laboratory errors occur in poorly standardized or manual processes. Preanalytical mistakes account for up to 70 % of total laboratory errors. The magnitude of the effect of these errors on patient care is not negligible, since information provided by clinical laboratories affects up to 60-70% of clinical decisions. The current focus of health care institutions on improving patient safety has given rise to a renewed interest in the preanalytical phase. Improvement of preanalytical processes currently constitutes a challenge to be faced by clinical laboratories. While quality control systems designed to ensure the quality of the analytical phase are highly developed and in use at most clinical laboratories, this is not the case for the preanalytical phase. One reason may be that laboratory professionals have always considered the analytical phase (but not the preanalytical phase) to be the most important process in their profession. Outsourcing of the sampling process could be another cause. Both of these factors have resulted in a decline in quality, as evidenced by an increase in preanalytical errors, and in turn, these errors have required that quality control systems be