Most algae do not use silicon in any form with one notable exception, diatoms. Silicon is a major constituent of diatoms. Diatoms are characterized by high growth rates and are often one of the key groups in forming algal blooms in natural waters, and as such it is an interesting group for cultivation. In this chapter we present methods for determining dissolved silica (DSi) and biogenic silica (BSi), oxide forms of silicon, based on colorimetric methods. BSi is determined after filtration and alkaline digestion.