An investigation of the phenomenon of open education (OE) was based on three questions. To answer why OE has received so much attention, researchers surveyed and analyzed arguments that influenced thinking about OE and establishment of OE institutes. To answer the question concerning the defining characteristics of OE, researchers took the following steps: investigated how the concept of OE was defined in publications that laid the foundation for the establishment of the Open University of the Netherlands (OU); analyzed publications about institutes of OE and about OE in general with regard to the manner in which the concept is used; and conducted a survey among personnel at the OU. Results showed very little agreement concerning the precise implications of the concept. Researchers developed an integrated model and instrument for analyzing OE. To answer the question concerning how open the OU is, researchers applied the integrated model and instrument. A second part of the study defined basic concepts of a general didactic nature. General didactic and educational literature and the didactic models used were investigated. Since existing models and definitions were not sufficiently comprehensive, central concepts were defined so that they applied to open higher education for adults. A didactic model was developed that placed emphasis on the didactic functions that must be fulfilled in education and on the manner in which these functions must be embodied by the education and the student. (Contains 98 references.) (YLB)