This thesis consists of four parts, and is based on a survey of the helminth parasites of Hertfordshire birds. Part I. (Section 1), deals with four species of trematodes found in Hertfordshire birds, Lyperosomum longicauda Rudolphi, 1809, Lutztrema monenteron Price & McIntosh, 1935, Dicrocoelioides petiolatum Railliet, 1900 and Brachylaemus fuscatus (Rudolphi, 1819) var. nicolli (Witenberg, 1925). 14 now host records and 1 new record for this country are confirmed. Part I. (Section 2), A new trematode, Reesella doviensis gen, ct sp. nov., from the Oystercatcher is described. (Reprint from J. Helminth. , 30, 81-86. ) Part II deals with sixteen species of cestodes recovered from Hertfordshire birds. Descriptions are given of Choanotaenia unicoronata Fuhrmann, 1908, Dilepis undula Schrank, 1788, Anomotaenia constricta Molin, 1858, A. borealis. Krabbe, 1869, Paricterotaenia parina Dujardin, 1845, Anonchotaenia globata von Linstow, 1879, Hymenolepis serpentulus Rudolphi, 1810, H. stylosa Rudolphi, 1809, H. amphitricha Rudolphi, 1819, H. farciminosa. Goez, 1782, H. fringillarum Rudolphi, 1809, Aploparaksis dujardinii Krabbe, 1869, and Raillietina (Skrjabinia) bonini Megnin, 1899. Three new forms are also described, Anomotaenia verulamii n. sp. from the mistlethrush, Paricterotaenia albani n. sp. from the starling and P. mariae n. sp. from the robin. A total of 13 new host records and 22 new records for this country are confirmed. Part III, (Section 1). deals with seven species of nematodes found in Hertfordshire birds. Descriptions are given of Porrocaecum ensicaudatum Zeder, 1800, Syngamus trachea Montagu 1811, S. merulae Baylis, 1926, Capillaria ovopunctata von Linstow, 1873, C. resecta Dujardin, 1845, C. colubae Rudolphi, 1819, and C. exilis Dujardin, 1845. One new host record and two new records for this country were confirmed. Part III. (Section 2), Descriptions are given of nine further species of Capillaria recovered from birds in Great Britain. They are Capillaria anatis Schrank, 1790, C. contorta Creplin, 1839, C. logicollis. Mehlis, 1831, C. nyrocinarum Madsen, 1945, C. obsignata Madsen, 1945, C. retusa Railliet, 1895, C. spinulosa von Linstow 1803, C. tenuissima Rudolphi, 1803, and C. triloba von Linstow, 1875; Four new host records and ten new records for this country were confirmed, Part IV. The results of a survey of the helminth parasites of Hertfordshire birds are tabulated. A total of 571 birds representing 22 species were examined. Of the birds examined 396 (69.3%) were found to be infected with helminths. During the survey 28 new host records and 25 new records for this country were confirmed. A total of 32 new host records and 35 new records for Great Britain are recorded in this thesis.