This chapter surveys the role of electrocardiography (ECG) in disease diagnosis, disease monitoring, and risk stratification in children and young adults with cardiomyopathy. In hypertrophic cardiomyopathy (HCM), left ventricular (LV) mass measured by magnetic resonance imaging correlates best with ECG measures when indexed to body surface area, with the 12-lead QRS amplitude-duration product (12-lead prod) having a correlation coefficient of 0.70 with total LV mass (p<0.0001), and 0.77 with LV mass indexed to body surface area (p<0.0001). Extreme values of 12-lead prod often signal unusual molecular etiology. In individual patients, changes in 12-lead prod follow changes in LV mass estimates well. In dilated cardiomyopathy (DCM), the LV mass lacks a significant correlation with 12-lead prod, but gradual reduction in voltages and broadening of QRS complex indicate progression of disease. Risk stratification for sudden death based on ECG measures for HCM (https://www.ecgriskscore.org) or a mixture of ECG and clinical parameters in other cardiomyopathies (DCM-SVA risk and https://arvcrisk.com/) is reviewed.