This paper reports a fundamental investigation on the aerodynamics of two-dimensional flapping wings in tandem configuration in forward flight. Of particular interest are the effects of phase angle (φ) and center-to-center distance (L) between the front wing and the rear wing on the aerodynamic force generation at a Reynolds number of 5000. Both experimental and numerical methods were employed. A force sensor was used to measure the time-history aerodynamic forces experienced by the two wings and digital particle image velocimetry was utilized to obtain the corresponding flow structures. Both the front wing and the rear wing executed the same simple harmonic motions with φ ranging from -180° to 180° and four values of L, i.e., 1.5c, 2c, 3c, and 4c (c is the wing chord length). Results show that at fixed L = 2c, tandem wings perform better than the sum of two single wings that flap independently in terms of thrust for phase angle approximately from -90° to 90°. The maximum thrust on the rear wing occurs during in-phase flapping (φ = 0°). Correlation of transient thrust and flow structure indicates that there are generally two types of wing-wake interactions, depending on whether the rear wing crosses the shear layer shed from the front wing. Finally, increasing wing spacing has similar effect as reducing the phase angle, and an approximate mathematical model is derived to describe the relationship between these two parameters.