The geomagnetic field acts both as an umbrella, shielding us from cosmic radiation, and as a window, offering one of the few glimpses of the inner workings of the Earth.

Ancient records of the geomagnetic field can inform us about the geodynamics of the early Earth and changes in boundary conditions through time.

Thanks to its essentially dipolar nature, the geomagnetic field has acted as a guide, pointing to the axis of rotation, thereby providing latitudinal information for both explorers and geologists.

A complete understanding of the geomagnetic field requires not only a description of the direction of field lines over the surface of the Earth but also information about its strength.

While directional information is relatively straightforward to obtain, intensity variations are much more difficult and are the subject of this chapter.

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In his treatise, De Magnete, published in 1600, William Gilbert described variations in field strength with latitude based on the sluggishness or rapidity with which a compass settled on the magnetic direction.

Magnetic intensity was first measured quantitatively in the late 1700s by French scientist Robert de Paul, although all records were lost in a shipwreck.

Systematic measurement of the geomagnetic field intensity began in 1830 (see, e.g., Stern, 2003 for a review).

Despite studies of the geomagnetic field that included some mention of its strength, stretching back to at least the time of Gilbert, basic questions such as what is the average field strength and whether there are any predictable trends remain subject to debate.

To study field intensity in the past requires us to use ‘accidental’ records; we rely on geologic or archaeological materials, which can reveal much about the behavior of the Earth's magnetic field in ancient times.

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There have been several fine reviews of the field of paleointensity (see, e.g., Valet, 2003) and the subject is developing very rapidly. Paleointensity data derived from archaeological materials will be considered elsewhere (Chapter 5.09).

This chapter will review the theoretical basis for paleointensity experiments in igneous and sedimentary environments especially with regard to experimental design.

We will then turn to new and updated existing databases.

Finally, we will highlight current topics of interest.