Our aim was to provide a better understanding of why many adults fail to receive recommended immunizations. Consumers (N = 2,002) and healthcare providers (N = 200) completed structured telephone interviews concerning their attitudes and knowledge about adult vaccines and factors affecting their vaccination decisions. Self-reported immunization rates for tetanus, influenza, and pneumococcal vaccines (which were emphasized in the surveys) were lower than goal rates set by national guidelines. Among the most common reasons consumers gave for not receiving immunizations were lack of physician recommendations and mistaken assumptions (e.g., healthy people do not need immunizations). Healthcare providers tended to cite concerns such as side effects, fear of needles, and lack of insurance coverage as reasons consumers forego vaccination. Providers also cited practice issues, such as lack of an effective reminder system, as barriers to increasing adult immunization rates. We conclude that a better understanding of why adults do not get vaccinated is important for efforts to increase immunization rates in this broad age group.