This thesis describes my study of situation awareness assessment of Vessel Traffic Service (VTS) operators. VTS operators are the traffic controllers on the water. They are responsible for a safe and efficient handling of vessel traffic. They monitor traffic, provide information on request and coordinate movement of ships in (emerging) conflict situations in harbours, rivers, and approach areas all around the world. Situation awareness is a term describing a person’s internal mental model of the current situation. Situation awareness is a concept that has been applied with success in many domains where controllers need to maintain an overview of their work. The concept of situation awareness presents new perspective for the study of VTS operator performance by investigating the mental picture of the operator. This perspective may lead to better understanding of the performance of VTS operators. The main objective of this thesis is to study how situation awareness of VTS operators can be assessed and to determine if the use and the assessment of VTS operator situation awareness leads to a better understanding of the performance of VTS operators. The thesis presents a framework for the examination of the concept situation awareness and for methods for assessing it in the context of VTS operator performance. This evaluation is based upon a literature study on situation awareness and VTS research and upon observations on VTS posts in the Netherlands. Two methods to assess situation awareness are developed. The first method, called SATEST, is derived from the SAGAT method developed by Mica Endsley and can be considered a traditional method for measuring situation awareness. This method presents a VTS scenario in an experimental setting. At unannounced, but previously specified times during these scenarios the system displays are blanked and the simulation is stopped. The operators are requested to answer a number of questions about their perception of the situation at that time, designed to reveal their situation awareness. After completion of the query, the simulation continues. SATEST was developed, tested and used in two experiments; in Rotterdam and Helsinki in the COMFORTABLE project. These experiments showed that SATEST was very useful in providing understanding of the VTS-work, the concept of situation awareness and its application in a VTS context. They also revealed the limitations of the method. It became clear in working with SATEST that the method covers situation awareness at any particular moment very well, but it is not very suitable to observe how situation awareness develops in a situation. The conclusion of the work with SATEST was that situation awareness is a useful concept in describing the work of the VTS operator, but that the method for assessing situation awareness needed more work. In the discussions of SATEST the idea grew that communication might be used to analyse when operators become aware of conflicts and when they will solve them. The second method, called PMI-P (Performance Measuring Instrument – Performance module), uses the assessment of communication to assess performance and situation awareness. Communication is scored on a PMI-P score form. This score form was constructed using the concept of Required Situation Awareness (RSA), developed in this thesis. RSA refers to the situation awareness needed to correctly handle a situation. For every event in the scenario the PMI-P score form contains information that the operator needs to communicate, and additional information that an operator may want to communicate. Communication between VTS and traffic is regarded as a direct measure of VTS operator performance and an indirect measure of situation awareness. The PMI-P score form captures the development of VTS operator situation awareness by describing all possible communications that an operator can have during the scenario run. This method was used in a project for the Rotterdam Port Authority. The results show that PMI-P is a powerful tool for assessing VTS operator performance. Understanding communication in PMI-P provides a solid handle on the assessment of VTS operator performance. This leads us to the question whether situation awareness is still needed as a concept in the assessment. The short answer to this question is yes: The required situation awareness of a situation that an operator requires has been used in the development of PMI-P to understand the scenario and to capture the required communication. Together the two methods offer a wide range of opportunities to study situation awareness of VTS operators, to study impact of new technology on VTS operator situation awareness, and to training operators.