This work analyses the effects of fragmentation on the landscape, vegetation structure and the vertebrates of Iberian fragmented forests, with particular emphasis on Holm oak (Quercus ilex) woods (Chapter 1). Intensive studies were performed on the effects of fragmentation on meduim-sized mammals (lagomorphs, carnivores and ungulates), as well as on territorial bird species whose spatial scale of distribution could be analysed properly within the extensive approach of the study (doves and pigeons, woodpeckers and passerines). Research was conducted in six Holm oak archipelagos (groups of forest remnants resulting from the process of forest fragmentation): four of them are located in the northern plateau of the Iberian peninsula and the remaining two in the southern plateau. In addition, two archipelagos of pine (Pinus pinea, P. Pinaster and P. Halepensis) plantations (one in each plateau), a sub-Cantabrian Pyrenean oak (Quercus pyrenaica) archipelago, and a mixed deciduous archipelago (Quercus robur, Quercus faginea, Fraxinus spp.) located in the Alava plains (northern Spain) were also considered to compliment the study. Overall, the vegetation and vertebrate fauna were sampled in 367 forest fragments, out of which 214 were Holm oak woods, 91 were pine plantations, 38 were Pyrenean oak woods, and 24 were mixed deciduous woods (Chapter 2). The analysis of the landscape structure (Chapter 3) revealed that the fragmentation process is in an advanced stage in the Holm oak woodlands of the Iberian plateaux. Almost 65% of woodlots are less than 2 ha in extent, so that they predominate over larger woods. Further, forest fragments are isolated from large woodlands, as shown by the fact that the nearest wood larger than 100 ha is generally found several kilometres apart from any given fragment. The vegetation of the Holm oak woodlands of the Iberian plateaux is generally degraded (little vertical development and low tree cover), although changing trends were found in relation to fragment size (Chapter 3). These trends, which were the same for both plateaux, consist of increases in shrub covers of herbs and trees as fragment size increases. It is suggested that such trends could be due to complex interactions between environmental factors and human pressure which, while intense in every case, vary with the fragment size. The development of the vegetation and and its degree of conservation and are better in the Holm oak woodlands of the northern plateau than in the woodlands of the southern plateau, so that northern fragments seem to provide habitats of better quality for forest vertebrates. The fragmentation of Holm oak woods entails negative effects on most species of forest birds (Chapter 4). Birds are most heavily affected by Holm oak wood fragmentation on the southern plateaux. This fact supports the hypothesis of a lower habitat quality of southern woodlots as compared to woodlots located on the northern plateaux. Further, only one species was found nesting on exclusively in the southern Holm oak woods, while 12 out of the 27 species (44%) recorded nesting in the northern Holm oak woods were not found in the south at all. Incidence functions (proportion of occupied woodlots in relation to woodlot size) revealed that the territorial requirements of most species are not satisfied by the smallest, but currently commonest, fragments (100 ha) fragmets, although such large fragments are now extremely uncommon. A acceptable proportion (at least 20%) of fragments with intermediate sizes hosts breeding individuals from 50% of the forest bird species found. Such medium-sized fragments account for more than 30% of the Holm oak woodlands currently found in the Iberian plateaux. Pine plantations are widespread forest habitats on both Iberian plateaux, and its extent has clear prospects of increasing in the near future, so that its analysis is particularly relevant in