Improved flow control valves are disclosed which are exceedingly accurate in maintaining a constant predetermined flow rate regardless of inlet and outlet pressures. The valves are preferably designed to enable them to be fabricated almost entirely from suitable polymeric composition materials. One embodiment disclosed is extremely well suited for ultra-pure applications which require extreme care to avoid any possible contamination of the liquid flowing therethrough. Another embodiment is disclosed which is designed to achieve substantially zero leakage flow thereby rendering it particularly well suited for use in applications requiring exceedingly precise flow control and in particular relatively low flow rate applications wherein leakage flow is of greater significance.