Hydrophilic linkers are useful for linking drugs to cell-binding ligands in ligand-drug conjugates, such as antibody-drug conjugates. The ligand-drug conjugate includes a cell-binding ligand capable of binding to a particular cell population, and a drug connected to the ligand by a hydrophilic linker. The hydrophilic linker includes one or more hydrophilic groups that render the linker hydrophilic. The hydrophilic linker may also include functional groups at the two termini for coupling to the drug and the cell-binding ligand respectively.