An injection device for use in injecting a fluid into a downhole target location , comprising: a housing defining an inlet for communicating with an injection line , an outlet for communicating with a target location , and an injection fluid flow path extending between the inlet and outlet ; and a valve assembly located within the housing and configured to control the flow of injection fluid along the flow path , the valve assembly comprising first and second valve members both arranged to move within the housing ; wherein the second valve member defines a flow path therethrough which forms part of the injection fluid flow path through the housing ; and the second valve member comprises or defines a piston arrangement configured to be actuated by fluid pressure to move said second valve member ; wherein the valve assembly comprises a biasing arrangement configured to bias the second valve member in a desired direction the valve assembly includes a limiting arrangement which is configured to limit movement of the first valve member during opening of the valve assembly; wherein a limiting feature is fixed relative to the housing , and a corresponding limiting feature is fixed relative to the first valve member ; wherein in an open configuration to permit flowthe first and second valve members are disengaged, and in a closed configuration to prevent flow the first and second valve members are engaged, and the first valve member is at least partially arranged inside the second valve member or the second valve member is at least partially arranged inside the first valve member when in the closed configuration; and the first valve member is biased against the second valve member by inlet fluid pressure (Pi) to assist sealing therebetween; wherein in the closed configuration, with the first and second valve members engaged, the corresponding limiting features of the limiting arrangement are separated. a housing defining an inlet for communicating with an injection line , an outlet for communicating with a target location , and an injection fluid flow path extending between the inlet and outlet ; and a valve assembly located within the housing and configured to control the flow of injection fluid along the flow path , the valve assembly comprising first and second valve members both arranged to move within the housing ; wherein the second valve member defines a flow path therethrough which forms part of the injection fluid flow path through the housing ; and the second valve member comprises or defines a piston arrangement configured to be actuated by fluid pressure to move said second valve member ; wherein the valve assembly comprises a biasing arrangement configured to bias the second valve member in a desired direction the valve assembly includes a limiting arrangement which is configured to limit movement of the first valve member during opening of the valve assembly; wherein a limiting feature is fixed relative to the housing , and a corresponding limiting feature is fixed relative to the first valve member ; wherein in an open configuration to permit flowthe first and second valve members are disengaged, and in a closed configuration to prevent flow the first and second valve members are engaged, and the first valve member is at least partially arranged inside the second valve member or the second valve member is at least partially arranged inside the first valve member when in the closed configuration; and the first valve member is biased against the second valve member by inlet fluid pressure (Pi) to assist sealing therebetween; wherein in the closed configuration, with the first and second valve members engaged, the corresponding limiting features of the limiting arrangement are separated. <EOS>