Salivary adenoid cystic carcinoma (ACC) and pleomorphic adenoma (PA) are the most common types of salivary gland tumours; the former is malignant and the latter is benign but with features of a border tumour. Proteoglycans (PGs) produced by neoplastic myoepithelial cells are ubiquitous in both types of tumours. However, normal myoepithelial cells of salivary glands do not have the ability to secrete PGs. When the synthesis of PGs is blocked, the pulmonary metastasis and perineural growth of salivary ACC as well as the implanting growth of salivary PA are inhibited, highlighting the important functions of PGs in the tumourigenesis and development of these two tumours. In this review, we summarise literature from the past 40 years, including more recent findings from our laboratory, to clarify the pivotal roles of PGs produced by neoplastic myoepithelial cells in both the histogenesis and biological behaviours of ACC and PA. Keywords: adenoid cystic carcinoma, pleomorphic adenoma, proteoglycans, RNA interference, xylosyltransferases