Sow: How deep do I sow my seeds?

Most seeds require oxygen to germinate, if buried too deep in their growing medium, or if the medium is too wet, the seeds may not get the oxygen they require.
Some seeds need to be in the light (surface sown) or in the dark (sown deep enough to receive little or no light) to germinate. A rule of thumb is to cover the seeds their own width deep in the growing medium, but some seeds prefer to be sown much deeper, and some fairly large seeds like to be surface sown (or higher).
Many seeds germinate best at certain temperatures, some will germinate at a comparatively wide range of temperatures, yet others need fluctuating temperatures.
Almost all seeds are waiting in a dormant state for some outside stimulus to break their dormancy, some just need sufficiently high ambiant humidity, others need scarification, vernalization or to be passed through the intestines of an animal.