This book presents a survey of the current state of knowledge of the potential impacts of climate change and variability on aeroallergens like pollen, mould, and indoor allergens in the United States and the allergic diseases associated with them. Allergies are prevalent in the United States and impose substantial economic and quality-of-life burdens. A recent nation-wide survey reported that 54.6% of people in the United States test positive for one or more allergens. Among specific allergens, dust mites, rye, ragweed, and cockroaches caused sensitisation in approximately 25% of the population. Allergies are the sixth most costly chronic disease category in the United States, collectively costing the health care system approximately $21 billion annually. The three main allergic diseases that have been associated with exposure to aeroallergens are allergic rhinitis (hay fever), asthma, and atopic dermatitis (eczema), which individually and collectively impose both substantial health effects and large economic burdens. The direct medical costs of asthma and allergic rhinitis (hay fever) are estimated to be $12.5 billion and $6.2 billion per year, respectively, and the direct medical costs of atopic dermatitis (eczema) are estimated to be $1.2−$5.9 billion per year. While data suggest that aeroallergen levels have remained relatively stable, the prevalence of allergic diseases in the United States has increased over the last 30 years, a trend that appears to be mirrored in other countries as well. The causes of this upward trend are as yet unclear. Because the economic impacts of allergic diseases associated with aeroallergens and the quality-of-life impacts on those individuals who suffer from them are already substantial, any climate change-induced enhancement or continuation of this trend in the United States would be of particular concern.