This chapter provides a summary of the landmark study known as the HACA Trial. Does mild therapeutic hypothermia improve neurologic outcomes compared with standard care normothermia in patients surviving ventricular fibrillation or pulseless ventricular tachycardic arrest? Starting with that question, it describes the basics of the study, including funding, study location, who was studied, how many patients, study design, study intervention, follow-up, endpoints, results, and criticism and limitations. Among the results, the preponderance of data suggests that temperature is an important variable for neurologic recovery after cardiac arrest. Guidelines suggest maintaining a target temperature between 32ºC and 36ºC for at least 24 hours after achieving target temperature. The chapter briefly reviews other relevant studies and information, discusses implications, and concludes with a relevant clinical case.