More attention is paid to the quality of models along with the growing importance of modelling in software development. We performed a systematic review of studies discussing model quality published since 2000 to identify what model quality means and how it can be improved. From forty studies covered in the review, six model quality goals were identified; i.e., correctness, completeness, consistency, comprehensibility, confinement and changeability. We further present six practices proposed for developing high-quality models together with examples of empirical evidence. The contributions of the article are identifying and classifying definitions of model quality and identifying gaps for future research.