Proposed and tested is a causal process that could account for empirical relationships between television viewing and academic achievement. It is argued that television, when used as a secondary activity, interferes with performance on otherwise intellectually demanding tasks. Performance on seven different cognitive processing tests were examined for respondents in four television-viewing conditions and a no-TV control group. Dependent variables included measures of short-term memory, linguistic processing speed, reading comprehension, complex problem-solving abilities, and mental flexibility. Predictions based on four mechanisms were tested. Significant performance decrements in television conditions occurred for measures of reading comprehension, spatial problem solving, and cognitive flexibility. Results were most consistent with the idea that background television influences performance by causing cognitive processing capacity limits to be exceeded on difficult and complex tasks.