Deep pectoral myopathy (DPM) and dorsal cranial myopathy (DCM) have been reported at different processing plants worldwide, causing economic losses due to condemnation or downgrading of carcasses. Both myopathies have split fibres and are associated with ischaemic necrosis, which may indicate a possible adaptive physiological response of fast-growing broilers. The aim of this chapter is, therefore, to summarise the most relevant findings concerning DPM and DCM regarding their aetiologies, macroscopic and microscopic characteristics of each disease and several associated factors, such as genetic lineage, sex, age, slaughter weight, nutritional deficiencies, bird mobility/exercise, rearing conditions, environmental factors, immunity status and stress.