Problematic harmful algal bloom is wide and tenacious, upsetting estuaries, coasts, and freshwaters system throughout the ecosphere, alongside disturbing human health, social life as well as national economy. Particular environmental factors supports growth of algal blooms, temperature always is significant when speaking about water-ecosystem. Disparity in temperature also found to affect the interaction of physical, chemical and biological parameters so it is equally imperative to consider effects of climate change, as change in climatic conditions supports unwanted growth of algae. Also inconsistency in climate equally contributes to the apparent increases of HAB, therefore effects of climate change needs to be totally comprehended along with development of the risk assessments and effective management of HABs. Increased HAB activities have a direct negative effect on ecosystems and they can frequently have a direct commercial impact on aquaculture, depending on the type of HAB. Causing economic impact also, as there is still insufficient evidence to resolve this problem. Therefore this chapter considers the effects of past, present and future climatic variability on HABs along with impacts of toxins release by them, on marine organism as well as human beings correspondingly, mitigation of HAB with help of suitable biological agents recognized.