Objectives: This study sought to estimate the temporal development in rates and incidences of appropriate and inappropriate implantable cardioverter-defibrillator (ICD) therapy and shocks by cardiac diagnosis in a real-world population of patients with secondary prevention ICDs. Background: Data on cardiac diagnoses and temporal development of ICD therapies in patients with secondary prevention ICDs are limited. Methods: Patients (N = 4,587) with a secondary prevention ICD were identified from the Danish Pacemaker and ICD Register (January 1, 2007, to December 31, 2016) and linked to nationwide administrative registers. The outcome of appropriate and inappropriate ICD therapy and all-cause mortality were analyzed by annual event rates, cumulative incidence plots, and Cox regression models. Results: During a mean follow-up of 3.6 ± 2.4 years, 1,362 patients (30%) experienced appropriate ICD therapy (16.8% shocks), and 350 patients (7.6%) experienced inappropriate ICD therapy (4.6% shocks). From 2007 to 2016, there was a significant temporal reduction in both appropriate and inappropriate ICD therapy from 28.2 (95% confidence interval [CI]: 21.6 to 37.0) to 7.9 (95% CI: 6.8 to 9.1) and 10.0 (95% CI: 6.4 to 15.5) to 1.0 (95% CI: 0.7 to 1.5) per 100 person-years (p for trends <0.001). Multivariate Cox regression analyses showed that arrhythmogenic right ventricular cardiomyopathy was associated with the highest probability of appropriate ICD therapy (hazard ratio: 2.45; 95% CI: 1.77 to 3.39; p < 0.0001), whereas patients with hypertrophic cardiomyopathy had the lowest probability (hazard ratio: 0.62; 95% CI: 0.42 to 0.93; p = 0.0196) when compared to patients with ischemic heart disease. Conclusions: In this nationwide real-life cohort of patients with secondary prevention ICDs, we observed a significant temporal decline in delivered appropriate and inappropriate shocks and ICD therapies in the last decade. A large proportion of patients still experienced ICD therapy but with significant differences by cardiac diagnosis.