Solid Phase Extraction (SPE) is the most widely used method for the extraction, changing of solvents, clean-up, concentration and fractioning of organic compounds from a number of samples. This procedure is also very useful for desalting proteins and sugar samples. However, most SPE procedures are still poorly developed, with little consideration to the physics involved in the process and are described as largely empirical, labour intensive and time consuming trial and error processes, without much systematization. This critical review aims to put forward a number of contradictions, disagreements, failings and shortcomings of the SPE procedures found in literature. The different arguments here introduced, attempt to challenge the suitability of this procedure, especially when it is carried out in the traditional way (under gravity and at high flow rate). The first part of the critical review focuses on describing the state-of-the-art SPE and is physical fundamentals of SPE.