Based on the Moment Generating Function (MGF), we derive closed form expressions for the Average Symbol Error Probability (ASEP) and the outage probability of the adaptive Decode and Forward (DF) relaying over independent and identical (i.i.d) Generalized-K channels for BPSK modulated signals. This kind of channel model includes jointly multipath fading and shadowing. It is very versatile and accurately approximates many of the commonly used channel models as Nakagami-m, Log-Normal distributions. Simulation results are discussed and proved to be in good agreement with our theoretical analysis.