WiFi offloading is a promising technique for addressing the mobile data explosion problem. In this paper, we consider two types of WiFi offloading techniques: 1) opportunistic WiFi offloading, where data offloading is conducted only when a mobile node opportunistically meets WiFi access points (APs); and 2) delayed WiFi offloading, where data transfer is delayed with the expectation of future AP contacts. We developed analytical models on WiFi offloading efficiency, which is defined as the ratio of the amount of offloaded data to the total amount of data. Extensive simulation results are given to validate the analytical models and to demonstrate the effects of the average cellular residence time, the delay bound, the variability of the WiFi residence time, the WiFi data rate, and the average session duration on WiFi offloading efficiency.