The microsomal triglyceride transfer protein is necessary for the assembly and secretion of lipoproteins containing apolipoprotein B. During the past year, significant progress has been made towards understanding the role of microsomal triglyceride transfer protein in lipoprotein assembly at both a cellular and molecular level. Studies carried out in a variety of heterologous expression systems, as well as the use of microsomal triglyceride transfer protein inhibitors in hepatoma cell lines, have been critical to this progress. It has been shown that microsomal triglyceride transfer protein plays a key role in the early stages of lipoprotein assembly, most likely by transferring lipid to nascent apolipoprotein B as it enters the lumen of the endoplasmic reticulum. The evidence indicates that microsomal triglyceride transfer protein does not play a major role in addition of bulk core lipid in the late stages of apolipoprotein B 48 lipoprotein assembly. Thus, microsomal triglyceride transfer protein appears to control the number of apolipoprotein B lipoprotein particles secreted but not the lipid composition.