**1. Introduction**

As a highly bioactive seaweed substance with many promising physiological activities, fucoidan has attracted attention from many industries all over the world. Even though fucoidans are a rich source of bioactive properties, the structural properties and bioactive mechanisms of fucoidans are poorly understood. Therefore, novel studies that either characterize the physical properties or biological activities of fucoidans will fill the knowledge gap between industrial applications and the scientific background of those applications.

Both purified and partially purified fucoidans isolated from brown seaweeds present high potential as preventative and therapeutic agents against a number of chronic diseases due to their anti-inflammatory, antioxidant, anticancer, neuroprotective, antiviral, antimicrobial, and anticoagulative properties.

This Special Issue is aimed at presenting updated information on well-documented studies of the structural characterization and major biological actions relevant for medical, cosmeceutical, and pharmaceutical applications that fucoidans isolated from brown seaweed can offer.
