A method of forming a porous PTFE membrane, the method comprising: forming an extruded PTFE polymer tape; transversely stretching the PTFE tape and drying to remove lubricant; longitudinally expanding the resulting tape; and then transversely expanding the longitudinally expanded tape as set out in one of (i)-(iii) (i) a longitudinal expansion ratio of 8:1 - 13:1 at a temperature of 340°C and a transverse expansion ratio of 32:1 - 34:1 at a temperature of 320°C; (ii) a longitudinal expansion ratio of 15:1 - 20:1 at a temperature of 345°C and a transverse expansion ratio of 22:1 - 30:1 at a temperature of 360°C; (iii) a longitudinal expansion ratio of 20:1 - 25:1 at a temperature of 340°C and a transverse expansion ratio of 20:1 at a temperature of 360°C; to thereby produce a membrane having a combination of permeability and porosity characterised by one or more of the following: permeability vs pore size equal to or below the line defined by log(Gurley Number)= 5.13 x 10 -3 (Bubble Point) - 1.26; wherein the Gurley Number is measured in accordance with the procedure outlined in the sections of the description entitled "Gurley Measurements" and "Frazier Measurements", and the Bubble Point is measured according to ASTM F3I 6-03; permeability vs pore size equal to or above the line defined by WEP = 3(Gurley Number) + 2500; wherein the water entry pressure is determined in accordance with the procedure outlined in the section of the description entitled "Water Entry Pressure Measurement"; permeability vs pore size equal to or above the line defined by log(Water Flux Rate) = -3.5 x 10 -3 (Bubble Point) + 1.3; wherein the water flux rate is determined in accordance with the procedure outlined in the section of the description entitled "Water Flux Rate Measurement"; permeability vs pore size equal to or above the line defined by log(water flux rate)= 16.9(mean flow pore size) - 1.85; wherein the mean flow pore size is determined in accordance with ASTM F3 I 6-03. forming an extruded PTFE polymer tape; transversely stretching the PTFE tape and drying to remove lubricant; longitudinally expanding the resulting tape; and then transversely expanding the longitudinally expanded tape as set out in one of (i)-(iii) (i) a longitudinal expansion ratio of 8:1 - 13:1 at a temperature of 340°C and a transverse expansion ratio of 32:1 - 34:1 at a temperature of 320°C; (ii) a longitudinal expansion ratio of 15:1 - 20:1 at a temperature of 345°C and a transverse expansion ratio of 22:1 - 30:1 at a temperature of 360°C; (iii) a longitudinal expansion ratio of 20:1 - 25:1 at a temperature of 340°C and a transverse expansion ratio of 20:1 at a temperature of 360°C; (i) a longitudinal expansion ratio of 8:1 - 13:1 at a temperature of 340°C and a transverse expansion ratio of 32:1 - 34:1 at a temperature of 320°C; (ii) a longitudinal expansion ratio of 15:1 - 20:1 at a temperature of 345°C and a transverse expansion ratio of 22:1 - 30:1 at a temperature of 360°C; (iii) a longitudinal expansion ratio of 20:1 - 25:1 at a temperature of 340°C and a transverse expansion ratio of 20:1 at a temperature of 360°C; to thereby produce a membrane having a combination of permeability and porosity characterised by one or more of the following: permeability vs pore size equal to or below the line defined by log(Gurley Number)= 5.13 x 10 -3 (Bubble Point) - 1.26; wherein the Gurley Number is measured in accordance with the procedure outlined in the sections of the description entitled "Gurley Measurements" and "Frazier Measurements", and the Bubble Point is measured according to ASTM F3I 6-03; permeability vs pore size equal to or above the line defined by WEP = 3(Gurley Number) + 2500; wherein the water entry pressure is determined in accordance with the procedure outlined in the section of the description entitled "Water Entry Pressure Measurement"; permeability vs pore size equal to or above the line defined by log(Water Flux Rate) = -3.5 x 10 -3 (Bubble Point) + 1.3; wherein the water flux rate is determined in accordance with the procedure outlined in the section of the description entitled "Water Flux Rate Measurement"; permeability vs pore size equal to or above the line defined by log(water flux rate)= 16.9(mean flow pore size) - 1.85; wherein the mean flow pore size is determined in accordance with ASTM F3 I 6-03. permeability vs pore size equal to or below the line defined by log(Gurley Number)= 5.13 x 10 -3 (Bubble Point) - 1.26; wherein the Gurley Number is measured in accordance with the procedure outlined in the sections of the description entitled "Gurley Measurements" and "Frazier Measurements", and the Bubble Point is measured according to ASTM F3I 6-03; permeability vs pore size equal to or above the line defined by WEP = 3(Gurley Number) + 2500; wherein the water entry pressure is determined in accordance with the procedure outlined in the section of the description entitled "Water Entry Pressure Measurement"; permeability vs pore size equal to or above the line defined by log(Water Flux Rate) = -3.5 x 10 -3 (Bubble Point) + 1.3; wherein the water flux rate is determined in accordance with the procedure outlined in the section of the description entitled "Water Flux Rate Measurement"; permeability vs pore size equal to or above the line defined by log(water flux rate)= 16.9(mean flow pore size) - 1.85; wherein the mean flow pore size is determined in accordance with ASTM F3 I 6-03.