How can I monitor the CPU temperature under linux?

As others have noted, you need the lm-sensors package. I thought I should mention that in addition to installing the package, you should run the sensors-detect command to identify the kernel modules that need to be loaded. If you haven't done this, manually run this once and check if it detects any sensors on board your computer. If you're lucky, your laptop will support an array of sensors. If not, you may still find a basic set of sensors for the CPU: