An impedance-based microbial growth detection method comprising the steps of: providing a vessel containing a liquid sample suspected of containing microorganisms disposed between and in contact with first and second electrodes wherein optionally the vessel is part of a sample container having one or more vessels; providing a time-varying electrical signal to the first electrode in contact with the liquid sample, wherein the second electrode is electrically connected to a phase-sensitive signal detector; and one of: a) selecting a frequency of said electrical signal so that an out-of-phase signal amplitude measured by said detector becomes equal to zero at the selected frequency, and monitoring the out-of-phase signal amplitude over time with the phase-sensitive signal detector, wherein an increase in the signal amplitude over time indicates microbial growth within the liquid sample; b) monitoring the out-of-phase signal amplitude over time with the phase-sensitive signal detector, determining a frequency at which the out of phase amplitude is zero by tuning a frequency of said electrical signal so that an out-of-phase signal amplitude measured by said detector becomes equal to zero; and repeating the determining step at predetermined time intervals; wherein an increase in said frequency over time is an indication of microbial growth within said liquid sample; or c) wherein the time-varying signal is generated by a voltage-controlled oscillator; and providing an integrated out-of-phase output signal of said detector into a frequency-control input of the voltage-controlled oscillator whereby the oscillator is tuned to a frequency at which an out-of-phase signal amplitude measured by the detector is equal to zero and wherein an increase in said tuned frequency over time indicates microbial growth within said liquid sample. providing a vessel containing a liquid sample suspected of containing microorganisms disposed between and in contact with first and second electrodes wherein optionally the vessel is part of a sample container having one or more vessels; providing a time-varying electrical signal to the first electrode in contact with the liquid sample, wherein the second electrode is electrically connected to a phase-sensitive signal detector; and one of: a) selecting a frequency of said electrical signal so that an out-of-phase signal amplitude measured by said detector becomes equal to zero at the selected frequency, and monitoring the out-of-phase signal amplitude over time with the phase-sensitive signal detector, wherein an increase in the signal amplitude over time indicates microbial growth within the liquid sample; b) monitoring the out-of-phase signal amplitude over time with the phase-sensitive signal detector, determining a frequency at which the out of phase amplitude is zero by tuning a frequency of said electrical signal so that an out-of-phase signal amplitude measured by said detector becomes equal to zero; and repeating the determining step at predetermined time intervals; wherein an increase in said frequency over time is an indication of microbial growth within said liquid sample; or c) wherein the time-varying signal is generated by a voltage-controlled oscillator; and providing an integrated out-of-phase output signal of said detector into a frequency-control input of the voltage-controlled oscillator whereby the oscillator is tuned to a frequency at which an out-of-phase signal amplitude measured by the detector is equal to zero and wherein an increase in said tuned frequency over time indicates microbial growth within said liquid sample. a) selecting a frequency of said electrical signal so that an out-of-phase signal amplitude measured by said detector becomes equal to zero at the selected frequency, and monitoring the out-of-phase signal amplitude over time with the phase-sensitive signal detector, wherein an increase in the signal amplitude over time indicates microbial growth within the liquid sample; b) monitoring the out-of-phase signal amplitude over time with the phase-sensitive signal detector, determining a frequency at which the out of phase amplitude is zero by tuning a frequency of said electrical signal so that an out-of-phase signal amplitude measured by said detector becomes equal to zero; and repeating the determining step at predetermined time intervals; wherein an increase in said frequency over time is an indication of microbial growth within said liquid sample; or c) wherein the time-varying signal is generated by a voltage-controlled oscillator; and providing an integrated out-of-phase output signal of said detector into a frequency-control input of the voltage-controlled oscillator whereby the oscillator is tuned to a frequency at which an out-of-phase signal amplitude measured by the detector is equal to zero and wherein an increase in said tuned frequency over time indicates microbial growth within said liquid sample.