Natural hydrogels are three-dimensional (3D) water-retaining materials with a skeleton consisting
of natural polymers, their derivatives or mixtures. Natural hydrogels can provide sustained or
controlled drug release and possess some unique properties of natural polymers, such as biodegradability,
biocompatibility and some additional functions, such as CD44 targeting of hyaluronic acid. Natural
hydrogels can be used with photosensitizers (PSs) in photodynamic therapy (PDT) to increase the range
of applications. In the current review, the pertinent design variables are discussed along with a description
of the categories of natural hydrogels available for PDT.