Monosacccharides are simple sugars with potentially free aldehyde or ketone groups and which cannot be broken down into smaller units by hydrolytic reactions. The simplest compound that can fit into the definition of a monosaccharide is glyceraldehydes, having an aldehyde group. Another simple form of monosaccharides is dihydroxyacetone, which has a ketone group. These compounds have three carbon atoms, and are hence called trioses.

The Structure of sugars was first elucidated by Emil Fischer a German chemist. He proposed that each monosaccharide can be projected in an open space chain having two enantiomorphic forms. For Example, glyceraldehyde can be projected in two forms:

The D-form will have the primary alcoholic group (OH) next to the bottom carbon atom on the right and in case of the L-form, on the left.
Monosaccharides also exhibit stereoisomerism which is based on the number of asymmetric carbon, hence can be projected in D and L forms. An aldotetrose (four carbon atoms) has two asymmetric carbon atoms with the possibility of forming four stereoisomers. Similarly an aldohexose (glucose) has four asymmetric carbon atoms which can have sixteen possible isomers.
     As for ketoses, most common sugar are ribulose, xylulose (ketopentose) and fructose (ketohexose). Some important ketoses are shown below.