The passenger level of service is a qualitative measure used to relate the quality of passenger services provided by the airport. The level of service can be perceived in correlation with both quantified and unquantified service characteristics. The global IATA Level of Service (LoS) standards are based on the maximum queuing time and space required per passenger for various terminal sub-systems. The provided level of service by the airport influences passenger experience and therefore is an important factor of airport attractiveness for airlines. The level of service is correlated with terminal facilities size so it is necessary to take it into account when designing new airports. Decrease of the LoS below acceptable standard is a trigger for operational and physical improvements of existing processes. Moreover defining the target level of service is necessary to determine terminal capacity and set coordination parameters. This paper discusses the importance of level of service at airport passenger terminals and presents methods for LoS assessment. Advantages and disadvantages were identified of each level of service research: manual observation, automatic detection, calculation and simulation. The paper includes recommendations for evaluating level of service during airport planning and operations phase.