A volcanic effusive event needs to be monitored to assess the possible impacted area. To achieve this, a number of measurements need to be made either from the ground, from the air, or from interpretation of satellite imagery. These measurements need to be made through established protocols so that derived parameters can be calculated and used to contribute to forecasting and mitigation measures. Key source term parameters include vent location, discharge rate, flow velocities (at-vent, down system, and flow front), feeder system, dimensions (thickness, length, width, underlying slope), feeder system thermal conditions (well-insulated or poorly insulated), and lava temperature, crystallinity and rheological state. First order needs are slope, discharge rate velocity, cooling rate, petrology and rheological characteristics. These are the source terms needed to assess and model lava flow hazard. In this chapter, we review the measurements that are typically made and the subsequent derivatives that can contribute to monitoring activities during an effusive event.