Reconfigurable optical switch apparatus comprising: m input ports each arranged to receive an optical signal having a respective one of y wavelengths; m output ports each arranged to output an optical signal having a respective one of the y wavelengths; k add ports each arranged to receive an optical signal to be added at one of the y wavelengths; k drop ports each arranged to output an optical signal to be dropped at one of the y wavelengths; and a switch matrix (20, 32), the switch matrix comprising m x k wavelength selective optical switches arranged in m rows, each connected between a respective input port and a respective output port, and k columns, each connected at one end to a respective drop port and at an opposite end to a respective add port, wherein each wavelength selective optical switch comprises a first port and a fourth port arranged in a respective said column and a second port and a third port arranged along a respective said row, each switch having an OFF state in which each optical signal having one of the y wavelengths received at the third port is output at the second port and each optical signal having one of the y wavelengths received at the fourth port is output at the first port and each switch having an ON state in which an optical signal having a preselected one of the y wavelengths received at the third port is output at the first port and an optical signal having said wavelength received at the fourth port is output at the second port, and in which each optical signal received which does not have the preselected wavelength is routed in the same manner as in the OFF state; and wherein the columns are grouped in adjacent pairs, in each pair a first column being connected to a respective drop port on a first side of the switch matrix and each wavelength selective switch in said first column having the fourth port arranged on said first side and a second column, adjacent the first column, being connected to a respective drop port on a second side, opposite the first side, of the switch matrix and each wavelength selective switch in said second column having the fourth port arranged on said second side, and wherein each of m, k and y is an integer which is at least 2. m input ports each arranged to receive an optical signal having a respective one of y wavelengths; m output ports each arranged to output an optical signal having a respective one of the y wavelengths; k add ports each arranged to receive an optical signal to be added at one of the y wavelengths; k drop ports each arranged to output an optical signal to be dropped at one of the y wavelengths; and a switch matrix (20, 32), the switch matrix comprising m x k wavelength selective optical switches arranged in m rows, each connected between a respective input port and a respective output port, and k columns, each connected at one end to a respective drop port and at an opposite end to a respective add port, wherein each wavelength selective optical switch comprises a first port and a fourth port arranged in a respective said column and a second port and a third port arranged along a respective said row, each switch having an OFF state in which each optical signal having one of the y wavelengths received at the third port is output at the second port and each optical signal having one of the y wavelengths received at the fourth port is output at the first port and each switch having an ON state in which an optical signal having a preselected one of the y wavelengths received at the third port is output at the first port and an optical signal having said wavelength received at the fourth port is output at the second port, and in which each optical signal received which does not have the preselected wavelength is routed in the same manner as in the OFF state; and wherein the columns are grouped in adjacent pairs, in each pair a first column being connected to a respective drop port on a first side of the switch matrix and each wavelength selective switch in said first column having the fourth port arranged on said first side and a second column, adjacent the first column, being connected to a respective drop port on a second side, opposite the first side, of the switch matrix and each wavelength selective switch in said second column having the fourth port arranged on said second side, and wherein each of m, k and y is an integer which is at least 2.