Modern mobile phones can support cellular and non-cellular wireless communications standards at the same time. Subsystems must operate in very close proximity to each other within a single device (in-device coexistence). The resulting high level of reciprocal leakage can at times cause considerable interference. This application note employs preliminary theoretical analysis and demonstrates how the problems of indevice coexistence can be measured. Exemplary, the effects of LTE and Bluetooth traffic on the WLAN performance of a commercially available smartphone are examined. These examples will also enable the reader to design tests for complementary scenarios.