The present monograph includes all the works carried out by the authors on Gremmeniella abietina, a major fungal pathogen which has caused very important losses on many conifer plantations in the North hemisphere. This fungal pathogen was found in diseased Pinus halepensis trees in a routine sampling carried out in Palencia, Northwestern Spain, in 1999. The unusual occurrence of this fungal species in a country where the ordinary climatological conditions (hot and dry summers) are not favourable for this fungus, in addition to the serious damages this fungus had caused in other parts of the world, caused a great concern on the future development of the disease in Spain. The lack of information available for the Spanish isolates of G. abietina, in addition to the great importance in terms of plant health of this major pathogen, prompted the necessity of carrying out different studies for a better understanding of the biology and behavior of G. abietina under the Spanish conditionsin order to be able of predicting potential disease outbreaks and spreading, and developing effective control strategies against the disease.