Diagnostic reasoning may relate to an established fault in a system or an established fault in an individual. A system is to be understood as a structured whole of components, while an individual is an object that is not analysed into components. With respect to systems, three types of diagnosis can be distinguished: non-explanatory, weak explanatory and strong explanatory. In section 2 we define these types, provide illustrations, and describe their respective functions. In section 3 we analyse the reasoning process by which non-explanatory diagnosis are constructed, and argue that the adaptive logic ALEXP, which makes a distinction between premises and expectations, is the best tool for modelling this kind of diagnostic reasoning. In section 4 we discuss (weak and strong) explanatory diagnostic reasoning, and show that it can be divided in three stages. Each stage must be modelled by means of a different adaptive logic. We will argue that the construction of weak explanatory diagnosis is some sort of abductive reasoning while the construction of strong explanatory diagnosis is some sort of 'inference to the best explanation'. In section 5 we discuss diagnosis and diagnostic reasoning in individuals. We show that non-explanatory diagnosis do not occur here, while the conclusions of 4 can be extended to individuals.