Boreal and temperate tree species are adapted to the seasonally varying climatic conditions with their annual cycle of development. The reason is that the frost hardy dormant phase and the susceptible growth phase are synchronized with the seasonality of the climate. The annual cycle includes various attributes such as timing of bud burst and other phenological events, and seasonality of photosynthetic capacity or frost hardiness of the trees. In this volume studies are viewed on dynamic ecophysiological models, which have increasingly been used during the last few decades, of the annual cycle of boreal and temperate tree species when projecting the ecological effects of climate change. Moreover, some additional new ideas on this topic are also introduced. A unifying notation is used throughout the book when discussing different aspects of the annual cycle. Main emphasis is on combining modelling with experimental studies and on the importance of the biological realism of the models.