This paper provides a broad overview of the various types of algae found in surface waters, their role in aquatic ecosystems, and the water-treatment problems they can cause. The focus is on tastes and odors and algal toxins. Algae can be regarded as "good" because they are the basis of the food chain. Some are "bad" when they grow excessively and create problems in water supplies and at the water treatment plant. The author ascribes the term "ugly" to certain algae whose physical form is not especially attractive, especially when they form blooms and scums in water supplies, but recognizes that beauty is in the eye of the beholder. Algae most often cause problems only when the aquatic ecosystem is unbalanced and one or more populations become over-productive. Typical water-treatment problems caused by excessive growth include clogging of intake screens, production of taste-and-odor compounds, poor flocculation and settling, and filter clogging. Blue-green algae produce taste-and-odor compounds that are offensive and difficult to remove; in addition, some blue-greens produce toxins. The subject of algal toxins is not new, but only since 1998 have they garnered attention in the United States because of their potential for causing human health effects. Diatoms and flagellated algae often produce taste-and-odor compounds that are more easily eliminated during water treatment than compounds produced by blue-greens, but these algae create problems at the water treatment plant other than odors, notably interference with clarification process and filter clogging. Excessive algal growth in impoundments elevates pH and increases dissolved oxygen to supersaturation levels during the daytime, both of which can cause water treatment problems. In addition, algal cells and the extracellular products they produce during photosynthesis increase levels of disinfection byproduct precursors in the raw water. When algae die and settle in thermally stratified impoundments, they decay and cause depletion of dissolved oxygen near the bottom and set the stage for the release from the lake sediments of iron, manganese, ammonia, and other substances that degrade water quality. Includes 38 references, table.