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There is growing concern about the impact of human activities on wildlife, both at the level of the individual animal and at a global population level, and the need for surveillance of wildlife for evidence of infectious and non-infectious diseases has never been greater. There is also much interest in attempting to help wildlife by treating and rehabilitating sick and injured wild animals and by providing supplementary feeding to garden birds. This thesis reviews the literature describing the diseases found in the United Kingdom (UK) in different birds of the orders Passeriformes and Columbiformes, the orders of birds with which members of the general public and wildlife rehabilitators are most likely to have contact. The thesis then collates and analyses the postmortem findings from wild bird surveillance carried out on 2048 birds of these orders at one diagnostic laboratory in Scotland over a twenty-year period (1994- 2013). The overall aim was to make maximum use of surveillance data already gathered but not previously readily available, to inform those involved with wildlife disease surveillance, wildlife rehabilitation, and members of the public providing supplementary feeding to garden birds. During the 20 years of wild bird disease surveillance, 42 endemic conditions or pathogens were identified, raising awareness and increasing our understanding of these conditions. One re-emerging disease, salmonellosis, came to prominence and then declined during the surveillance period, and was confirmed in approximately 350 garden birds. Two new conditions were described in finches; Escherichia albertii bacteraemia in approximately 150 finches and Trichomonas gallinae infection in approximately 370 finches. The large numbers of birds with salmonellosis, E. albertii bacteraemia or trichomonosis permitted further analysis by species of bird, geographic region, and distribution by age and sex, permitting conclusions to be drawn regarding the epidemiology of these diseases. Two new conditions were diagnosed in choughs (Pyrrhocorax pyrrhocorax), a species of conservation concern in the UK; a developmental abnormality of the eye and sometimes brain of young choughs, most likely inherited, and significant helminthosis caused by spirurid gizzard worms and intestinal thorny-headed worms. These findings will influence future attempts to conserve this species in Scotland. Another new condition encountered was enteritis and/or hepatitis associated with schistosome-like eggs, diagnosed in blackbirds (Turdus merula) and a dunnock (Prunella modularis). More specific identification of the causal organism and evaluation of potential zoonotic implications are required. Two conditions were investigated for which no satisfactory aetiological agent could be identified; a nonsuppurative encephalitis affecting multiple fledgling starlings (Sturnus vulgaris) and house sparrows (Passer domesticus), and a necrotic oesophagitis of unknown cause detected in five chough nestlings. Three organisms identified in wild birds elsewhere in the world and found for the first time in the UK as part of this surveillance study were the avian gastric yeast Macrorhabdus ornithogaster (“megabacteria”) in greenfinches (Chloris chloris) and a waxwing (Bombycilla garrulus), Mycoplasma sturni in blackbirds, starlings and corvids, and Ornithonyssus sp. mites in corvids. Screening for two zoonotic pathogens exotic to UK wildlife, highly pathogenic avian influenza virus (HPAIV) and West Nile virus (WNV) was carried out on over 600 samples and over 500 samples respectively, but no positive results were obtained. Investigation of novel and re-emerging conditions and screening for exotic pathogens relied heavily on work carried out by other laboratories, underlining the importance of collaboration between multiple laboratories when carrying out disease surveillance. To aid those working in wild bird disease surveillance, diagnosis and treatment, a collection of approximately 700 images of lesions, parasites and their eggs or oocysts is included as an appendix to this thesis, as has a guide to the presumptive identification of some of the internal parasites encountered. This study has demonstrated the ever-changing nature of diseases of wild birds of the orders Passeriformes and Columbiformes, and the same is likely to be true of wild birds in other orders. Continued wild bird disease surveillance is essential, to help safeguard the health of wildlife, livestock, humans, and indeed the environment itself.