This thesis is an investigation of the prevalence of avian adeno-associated virus (AAAV), chicken anaemia virus (CAV), fowl adenovirus (FAdV), and infectious bursal disease virus (IBDV), and risk factors for CAV, FAdV, and IBDV, among commercial broiler chicken flocks in Ontario. Biological samples from 231 randomly-selected flocks were collected at slaughter. Laboratory methods to determine flock exposure to these viruses included PCR and ELISA or AGID. A subset of FAdV- and IBDV-positive samples was genotyped. Data on management and biosecurity practices used in raising the study flocks were collected via a face-to-face interview with the producers. Associations of exposure to the three viruses or the presence of FAdV genotypes with management/ biosecurity practices were investigated using generalized estimating equations, linear regression, and logistic regression models. The flock-level period prevalence of exposure to AAAV, CAV, FAdV, and IBDV during grow-out were 88.76%, 77.06%, 96.54%, and 48.92%, respectively. Potentially pathogenic FAdV genotypes constituted 39.38% of the isolates. The most common IBDV genotype identified was IBDV NC171 (60%). Risk factors for CAV included: caked or matted litter under water lines; some feed mills; fall season; and use of mixed (fresh and leftover) feed. Protective factors for CAV included some other feed mills and increasing rest period. Risk factors for IBDV included: increasing annual production; grass around the barn; and thermal discomfort of the flock during grow-out. Protective factors for IBDV included: geographic location; fall season; some hatchery companies; barn wall constructed of concrete or concrete at the bottom and wood on top; increased frequency of washing barns with high pressure in the past year, and flushing water lines during grow-out. Risk factors for exposure of flocks to pathogenic FAdV genotypes included: grass around the barn; tunnel or other types (axis, natural, or mixed) of ventilation; immediate disposal of dead birds in restricted area, or in/ out of the controlled access zone; and thermal discomfort during grow-out. Protective factors included some hatchery companies and increasing duration of the brooding period. This study estimated baseline prevalence of exposure to AAAV, CAV, FAdV, and IBDV, and identified genotypes and risk factors to consider in disease control.