The present invention relates to amino acid sequences binding to serum albumin. In particular, the present invention relates to improved immunoglobulin single variable domains (also referred to herein as “ISV's” or “ISVD's”), and more in particular improved heavy-chain immunoglobulin single variable domains (also referred to herein as “ISV's” or “ISVD's”) binding to serum albumin, as well as to proteins, polypeptides and other constructs, compounds, molecules or chemical entities that comprise such improved serum albumin binders.