His research interests include information retrieval, machine learning techniques and applications and information privacy/security with more than 100 publications. In particular, he has recently worked on applications of detection and anonymization of sensitive text information, privacy-preserving information search and malicious software detection with information retrieval and machine learning techniques. Si is an associate editor of ACM Transactions on Information System (TOIS), ACM Transactions on Interactive Information Systems (TIIS) and an editorial board member of Information Processing and Management (IPM). He has been an Area Chair for ACM SIGIR, WWW, WSDM and CIKM. Si was a workshop co-chair for SIGIR 2011.