We econometrically evaluate information worker productivity at a midsize executive recruiting firm and assess whether the knowledge that workers accessed through their electronic communication networks enabled them to multitask more productively. We estimate dynamic panel data models of multitasking, knowledge networks, and productivity using several types of micro-level data: (a) direct observation of more than 125,000 email messages over a period of 10 months; (b) detailed accounting data on individuals' project output and team membership for more than 1,300 projects spanning five years; and (c) survey and interview data about the same workers' IT skills, IT use, and information sharing. We find that (1) more multitasking is associated with more project output, but diminishing marginal returns, and (2) recruiters whose network contacts have heterogeneous knowledge-an even distribution of expertise over many project types-are less productive on average but more productive when juggling diverse multitasking portfolios. These results show how multitasking affects productivity and how knowledge networks, enabled by IT, can improve worker performance. The methods developed can be replicated in other settings, opening new frontiers for research on social networks and IT value.