Due to the increase in air transport, there are some airports that become small. Its capacity is not enough for the current demand. In order to meet the current needs, expansions or changes are needed. This would be the case at Barcelona-El Prat airport, with the new terminal 1. In order to expand or change a terminal or an airport, manuals are used. These manuals provide guidelines to design them in an efficient and optimum way. In those manuals, there is an important parameter called Level of Service (LoS) which guarantees if the airport fulfils or not the basic needs. This project has two main objectives: · Introduce the Level of Service concept and verify and justify if the LoS's classification done in Airport Development Reference Manual's (ADRM) last Edition is better and more realistic in comparison with the previous one. · Determine if some of the terminal 1 subsystems at Barcelona-El Prat airport adapt to the new LoS's classification done in the 10th edition, because the current edition when the terminal was opened for the first time, in 2009, was the 9th. To accomplish them, first, a familiarization has been done with the concept of Level of Service. Both ADRM editions, writen by IATA, have been also analysed. Next, a comparison between both editions has been made applied at Barcelona-El Prat to determine the Level of Service. The subsystems analysed are the following: check-in desks zone, security control, passports' control (both outbound and inbound) and baggage claim. It has been verified that the 10th edition is more complete, realistic and effective, since, unlike the previous one, it considers two parameters: space per passenger and waiting time. Regarding the analysis carried out in the aforementioned facilities of terminal 1 at Barcelona-El Prat airport according to the tenth edition, it is concluded: · Security control and passports' control in arrivals have a sub-optimal level of service · Check-in desks zone and passports' control at departures are oversized · Baggage claim area has an optimum level of service Therefore, it can be concluded that both objectives have been fulfilled although it has been seen that LoS of some facilities is not appropriate according to the new classification.