Description Logics are commonly used for the development of ontologies. Yet they are well-known to present difficulties of comprehension, e.g. when confronted with the justification for a particular entailment during the debugging process. This paper describes a study into the problems experienced in understanding and reasoning with Description Logics. In particular the study looked at: functionality in object properties; negation, disjunction and conjunction in Propositional Logic; negation and quantification; and the combination of two quantifiers. The difficulties experienced are related to theories of reasoning developed by cognitive psychologists, specifically the mental model and relational complexity theories. The study confirmed that problems are experienced with functional object properties and investigated the extent to which these difficulties can be explained by relational complexity theory. Mental model theory was used to explain performance with negation and quantifiers. This suggests that Boolean logic is easier to assimilate in Disjunctive Normal Form than in other forms and that particular difficulties arise when it is necessary to backtrack to form a mental model. On the other hand in certain cases syntactic clues seemed to contribute to reasoning strategies.