Information security is receiving a great deal of attention as computers increasingly process more and more sensitive information. A multilevel secure database management system (MLS DBMS) is designed to store, retrieve and process information in compliance with certain mandatory security requirements, essential for protecting sensitive information from unauthorized access, modification and abuse. Such systems are characterized by data objects labelled at different security levels and accessed by users cleared to those levels. Unless transaction processing modules for these systems are designed carefully, they can be exploited to leak sensitive information to unauthorized users. This text covers research in developing secure transaction processing for popular MLS DBMS architectures, such as kernelized, replicated, and distributed architectures, and advanced transaction models such as workflows, long duration and nested models. Further, it explores the technical challenges that require future attention.