We will start by explaining which factors influence power consumption in CMOS chips. We will then explain a number of commonly used techniques to control the power versus performance balance such as clock and powergating, clock and supply voltage scaling and heterogenous multiprocessing. We will continue explaining how these techniques are implemented in linux using various frameworks such as cpuidle, cpufreq, common clock framework, runtime PM, generic powerdomains, device QoS constraints, system suspend. We will then discuss how application code affects the device power consumption and discuss how Android and Maemo handle this. We will also discuss various debugging mechanisms and tools such as powertop, ftrace and other information in linux sysfs and debugfs. We will also explain hardware techniques we have used to gain insight in the system operation beyond the view of software.