Although electron cyclotron resonance (ECR) plasma reactors are being used in the microelectronics industry for etching and deposition of thin films they are prone to instabilities that can wreak havoc with manufacturing processes. Plasma conditions are often unstable because of nonlinear coupling between wave propagation, power absorption, neutral density, and charge density profiles. We report on hysteresis, multiple steady states and abrupt transitions in an ECR plasma reactor that can alter the plasma properties drastically. Substrate platen floating potential and Langmuir probe measurements are used to identify several abrupt transitions and regions in the operating parameter space where the plasma can exist in either one of two different states under identical operating conditions (microwave power, pressure, flow rate, magnetic field, rf bias etc.) Depending on how the plasma is started either branch can be obtained. Abrupt transitions and hysteresis in plasma properties are observed both with and without rf bias under typical conditions used in plasma processes. Such transitions can cause drastic deviations in plasma parameters—density, current, uniformity—and may result in degraded process characteristics—uniformity, rate, selectivity, linewidth control. Statistical techniques used to design and develop processes cannot properly account for discontinuous changes in the state of the plasma and can produce brittle processes that can suddenly lead to catastrophic failure. Care must be taken to map out the regions of bistability for the purpose of avoiding them or controlling the plasma in these regions. While the mechanism responsible for the multiple steady states and abrupt transitions reported here is not well understood, it is clear that these phenomena depend critically on the neutral density.