Heterochromatin protein 1 (HP1) is an evolutionarily conserved chromosomal protein that binds lysine 9-methylated histone H3 (H3K9me), a hallmark of heterochromatin, and plays a crucial role in forming higher-order chromatin structures. HP1 has an N-terminal chromodomain and a C-terminal chromo shadow domain, linked by an unstructured hinge region. Although biochemical and structural studies have revealed each domain's properties, little is known about the mechanisms by which these domains cooperate to carry out HP1's function in forming higher-order chromatin structures. In this review, we summarize HP1's biochemical and structural properties and highlight the latest findings regarding HP1's interactions with nucleosomes.