Spaghetti meat (SM), woody breast (WB), and white striping (WS) are myopathies that affect the pectoral muscle of fast-growing broiler chickens. Although reported in numerous countries with intensive poultry production, no studies have characterized these myopathies in Canada. By obtaining samples from two large processing plants in Ontario, the objectives of this thesis were to 1) determine the prevalence of and identify the risk factors associated with SM, WB, and WS in Ontario broilers; 2) characterize the morphological changes associated with breast myopathies by evaluating the macroscopic and histological features of affected fillets; and 3) evaluate the transcriptomic profiles of normal and SM- or WB-affected fillets using RNA sequencing and droplet digital PCR. The prevalence of SM, severe WB, and mild or moderate WS was 36.3%, 11.8%, and 96.0%, respectively (n = 9,250). Most (85.1%) of the fillets presented with multiple myopathies. Regression analyses showed that the odds of SM increased with live weight and higher environmental temperature during the grow-out period. The odds of WB increased with live weight and flock mortality rate during grow-out. Macroscopically, the odds of SM and severe WB were significantly associated with increased fillet thickness and weight. Histologically, myopathies had overlapping lesions consisting of polyphasic myodegeneration, perivascular inflammatory cuffing, and endomysial accumulation of fibrous tissue and fat. Transcriptomic analysis showed that there were no differences between normal and SM fillets, while the transcriptomic profile of WB fillets significantly differed from both normal and SM. Most of the differentially expressed genes in WB fillets were involved with the extracellular environment and immune response. Taken together, our results show that the macroscopic and microscopic features of SM, WB and WS in our cohort were similar to those has been reported from other countries. These myopathies were highly prevalent in Ontario broilers, suggesting a significant economic impact on the Canadian poultry industry. While WS was present in most fillets, risk factor analysis showed that heavier broilers had an increased risk of both SM and severe WB. Lastly, lack of significant differences in expressed genes between normal and SM fillets suggests that SM may not be caused by factors intrinsic to the broiler breast muscle, but rather could be caused by mechanical disruption after slaughter.