The paper represents the concept map based adaptive knowledge assessment system. Advantages of concept maps are analyzed emphasizing that the approach offers a reasonable balance between requirements to assess higher levels of knowledge according to Bloom's taxonomy and complexity of a system. Concept maps allow revealing of student's knowledge structure, promote system thinking and support process oriented learning where a study course is divided into stages in each of which knowledge assessment is carried out. The developed knowledge assessment system consists from a teacher's, learner's and administrator's modules and is implemented as a multiagent system. Four prototypes of the system developed within four projects are described. The first prototype supports only fill-in-the-map tasks where a learner must put given concepts in correct places. The second prototype provides changing the degree of task difficulty, thus, performing adaptation to a learner's knowledge level. The set of tasks are also extended by construct-the-map tasks. Improvements implemented in the third prototype allow using of directed arcs and standard relationships in concept maps. The three-tier architecture used in the fourth prototype is chosen to rise the security level of the system. Besides that learner's support is considerably expanded giving help and tutoring to a learner. Results of evaluation of the developed system's prototypes in different study courses are presented. The paper concludes with the comparison of all four prototypes using all main characteristics of the developed knowledge assessment system.