Antibodies and antigen binding fragments that specifically binds to Siglec-15 are described herein. These antibodies or antigen binding fragments may have the ability of inhibiting differentiation of osteoclasts and/or the ability of inhibiting the bone resorption activity of osteoclasts. Compositions and cells expressing anti-Siglec-15 antibodies or antigen binding fragments are also disclosed herewith. Anti-Siglec-15 antibodies may also be useful for the treatment of bone loss, or bone diseases. Methods for the detection or diagnosis of bone loss or bone-related diseases are also described.