Shell integration (the shell method in integral calculus) is a means of calculating the volume of a solid of revolution, when integrating along an axis parallel to the axis of revolution. While less intuitive than disc integration, it usually produces simpler integrals.
It makes use of the so-called "representative cylinder". Intuitively speaking, part of the graph of a function is rotated around an axis, and is modelled by an infinite number of hollow pipes, all infinitely thin.
The idea is that a "representative rectangle" (used in the most basic forms of integration – such as ∫ x dx) can be rotated about the axis of revolution; thus generating a hollow cylinder. Integration, as an accumulative process, can then calculate the integrated volume of a "family" of shells (a shell being the outer edge of a hollow cylinder) – as volume is the antiderivative of area, if one can calculate the lateral surface area of a shell, one can then calculate its volume.
Mathematically, this method is represented by:
if the rotation is around the y-axis (vertical axis of revolution), or
if the rotation is around the x-axis (horizontal axis of revolution).
See also 
- CliffsNotes.com. Volumes of Solids of Revolution. 12 Apr 2011 <http://www.cliffsnotes.com/study_guide/topicArticleId-39909,articleId-39907.html>.
- Weisstein, Eric W., "Method of Shells", MathWorld.
- Frank Ayres, Elliott Mendelson:Schaum's outlines: Calculus. McGraw-Hill Professional 2008, ISBN 978-0-07-150861-2. pp. 244–248 (online copy at Google Books)