A downhole device comprising: a device body having a longitudinal axis, the device body adapted to be connected to the end of a tool string; and a guide member for assisting in progression of the tool string along a wellbore, the guide member comprising a first portion and a second portion, the first portion of the guide member being carried by the second portion of the guide member, wherein the guide member is rotatable with respect to the device body about the device body longitudinal axis, wherein the first portion of the guide member comprises a roller unit having a greater transverse outer dimension than the second portion of the guide member, wherein the roller unit extends at least substantially around the device body longitudinal axis, wherein the roller unit is rotatable with respect to the device body about an axis perpendicular to the device body longitudinal axis, wherein the first portion of the guide member defines a leading end of the downhole device, wherein the first portion of the guide member comprises first and second rollers (38, 40) and an axle, wherein the first and second rollers (38, 40) are symmetrically disposed around the device body longitudinal axis, wherein each roller (38, 40) comprises a projecting part (50, 52), the projecting part projecting perpendicular to the device body longitudinal axis, wherein the projecting part (50, 52) of each roller (38, 40) is an eccentrically-shaped part, the eccentrically-shaped part comprising an oval shape which extends from an outer diameter of the roller (38, 40) to the end of the projecting part, wherein outer edges (46, 48) of the first and second rollers (38, 40) define the leading end of the downhole device, and wherein the second portion of the guide member extends along the device body longitudinal axis. a device body having a longitudinal axis, the device body adapted to be connected to the end of a tool string; and a guide member for assisting in progression of the tool string along a wellbore, the guide member comprising a first portion and a second portion, the first portion of the guide member being carried by the second portion of the guide member, wherein the guide member is rotatable with respect to the device body about the device body longitudinal axis, wherein the first portion of the guide member comprises a roller unit having a greater transverse outer dimension than the second portion of the guide member, wherein the roller unit extends at least substantially around the device body longitudinal axis, wherein the roller unit is rotatable with respect to the device body about an axis perpendicular to the device body longitudinal axis, wherein the first portion of the guide member defines a leading end of the downhole device, wherein the first portion of the guide member comprises first and second rollers (38, 40) and an axle, wherein the first and second rollers (38, 40) are symmetrically disposed around the device body longitudinal axis, wherein each roller (38, 40) comprises a projecting part (50, 52), the projecting part projecting perpendicular to the device body longitudinal axis, wherein the projecting part (50, 52) of each roller (38, 40) is an eccentrically-shaped part, the eccentrically-shaped part comprising an oval shape which extends from an outer diameter of the roller (38, 40) to the end of the projecting part, wherein outer edges (46, 48) of the first and second rollers (38, 40) define the leading end of the downhole device, and wherein the second portion of the guide member extends along the device body longitudinal axis.