Â© 2018 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. Coastal ocean observing systems are used throughout the world to provide sustained, real-time observations of the coastal ocean environment, to address regional issues and to provide local context to global environmental variability. However, the level of sophistication in these systems is not equal globally mainly due to resource limitations. Over the past decade these systems have matured as platform, sensor and telecommunications technologies have improved, become more computationally and energy efficient, and have benefited from the development of new sensors. As the systems increase in sophistication and complexity, the applications of coastal observing systems continues to expand and have benefit for a diverse population of users and stakeholders. These include both short-term and long-term applications. Short-term applications include ocean weather prediction, coastal dynamics, search and rescue operations, navigation, fisheries, and the monitoring, assessment and prediction of ocean hazards such as ocean acidification, coastal zone hypoxia, harmful algal blooms, oil spill tracking and mitigation. In the long term, the observations are used for assessment of climate change variables, such as sea-level rise, changing ocean chemistry and biology (including ocean), and changes to regional meteorological wind and ocean circulation patterns. This article provides a summary of the motivation and rationale for coastal ocean observing systems in the context of global ocean observing initiatives followed by descriptions of the platforms and applications commonly employed by coastal systems that are currently deployed. Also presented is a discussion of the role of numerical modeling and prediction in the context of observational science. Lastly, we look to the future and challenges to the next generation of observational science, specifically the training of an ocean observing workforce, sustainability, and linking ocean observing activities with human health.