Turn now from the most general spacetime in full 3 + 1 dimensions to the special case of spherical symmetry. Why should we do this? Actually, spherical systems provide very useful computational, physical and astrophysical insight and working with them serves multiple purposes. The field equations reduce to 1 + 1 dimensions – variables may be written as functions of only two parameters, a time coordinate t and a suitable radial coordinate r – and are much simpler to solve in spherical symmetry. Solving them is a very cost-effective way of probing dynamical spacetimes with strong gravitational fields, including spacetimes with black holes. After all, nonrotating stars and black holes are themselves spherical, so many important aspects of gravitational collapse, including black hole formation and growth, can be studied in spherical symmetry. For example, the numerical study of spherically symmetric collapse to black holes led to the discovery of critical phenomena in black hole formation. The simplification in the equations, together with the reduction in the number of spatial dimensions, means that the system of spherical equations can be solved more quickly, in terms of both human input and computer time, and with much higher accuracy, than the set required for more general spacetimes. As a result, tackling problems in spherical symmetry provides an excellent starting point for learning how to do numerical relativity. It also serves as a convenient laboratory for experimenting with different gauge choices (coordinates) and for generating high precision, test-bed solutions for numerical codes designed to work in higher dimensions.