Chitosan is one of the most abundant marine-based biopolymers. Low molecular weight and oligomeric chitosans are water-soluble hydrolysates of chitosan. They have been shown to have a wide range of biological activities and industrial applications. In particular, low molecular weight and oligomeric chitosans have been reported to have the health benefits such as immunity regulation, anti-tumor, liver protection, blood lipids lowering, anti-diabetic, antioxidant and anti-obesity. In this paper, the preparation and analytical methods, and bioactivities of these low molecular weight and oligomeric chitosans were reviewed, with the latest progresses introduced and discussed.