[en] Having defined the decline concept, the reasons of loss of vitality for in¬digenous oaks are reviewed according to three types of factors (predisposing, inciting and contributing). The easy identifiable symptoms are early defoliation and discolouration which are regularly watched everywhere in the European Community. Various other symptoms could be noticed during researches carried out in Belgium. They are in relation with the vitality of the crown (twigs dying or fall, one-year twigfall), the ramification (reduced ultimate ramification, short internodes), the bark (desquamation, beetles galleries, presence of Armil¬laria, loosening, black dripping and fall), the reduced diameter increment and the roots (breakage, darkening). These symptoms are described and their possible origins discussed.
