Harmful Algal Blooms (HABs) are of great concern because of their toxicity and/or the damage they cause to ecosystems and coastal resources. Toxic HAB events in aquaculture can have adverse effects on economy (fish or shellfish mortality) and public health (human disease, mortality) and high-biomass, non-toxic blooms can affect water quality and tourism. Aquaculture and tourism are mainly located in coastal zones that can be affected by eutrophication, sometimes cited as a cause of HABs. Amongst the challenges related to HABs we can cite the need to: (i) understand the physiological/biological/environmental factors that regulate HABs and in particular the underlying physical-biological interactions, (ii) forecast HAB events, (iii), describe and understand the role of introduced species and (iv) assess the impact of climate change on HABs (occurrence/frequency/magnitude). Recent developments in automated observations techniques are providing new insight into dynamics of HABs. Models, providing that they are carefully validated and adapted to the situation (region/species), are necessary tools for forecasting, assessing the impact of future scenarios and for process studies. The development of HAB models requires interdisciplinary (biological, chemical, and physical) research. Strong interaction between modelers and experimentalists is crucial and data availability essential. Recognizing the rapid progress in observation techniques and HAB modeling in recent years, we invite contributions of modeling studies, laboratory and experimental research, field studies and remote sensing investigations that advance our ability to understand underlying physical-biological interactions that control HABs, to improve HAB model validation, to forecast HAB events, or to assess effect of climate change.