This textbook covers the fundamentals of fouling and scaling in reverse osmosis systems. It includes theory and practice of pre-treatment, fouling and scaling in reverse osmosis applied for drinking and industrial water production. The impact of the water source - seawater, river water, brackish groundwater and (treated domestic) waste water - will be discussed in depth. The book presents the knowledge and experience gained at IHE Delft over the last 25 years during the implementation of the master programme in Water Supply Engineering and during the implementation of state-of-the-art research in understanding and solving operational problems in full scale desalination plants. It presents the expert knowledge of IHE Delft in the areas of pre-treatment for reverse osmosis systems, assessment of water quality with respect to fouling potential, development of methods for quality assessment, modified fouling index ultrafiltration at constant flux, transparent exopolymer particles, antiscalant dose optimization, biological growth potential), algal blooms, scaling control. The book will be used in the annual master programme at IHE Delft and it will be of interest for students, academics, engineers and managers in drinking water facilities all over the world.