Expressions are presented for the 1/f current noise in bipolar transistors. These expressions are based on mobility-fluctuation 1/f noise. The current dependences of the 1/f noise intensities are found to be S_IC ≈ I_C and S_IB ≈ Iⁿ_B, where I_C is the collector current and I_B the base current. The exponent n is in the range of 0.5–2. An expression for the base current noise, induced by intrinsic base resistance fluctuations, is derived. The current dependence of the induced noise is S_IB ≈ I⁴ _B. 1/f Noise experiments are presented for silicon bipolar transistors. The experimental results can be explained in terms of fluctuations in the mobility of free charge carriers. Experimental results from literature on 1/f noise in transistors are also discussed here in terms of mobility fluctuations. The agreement between experiment and theory is often found to be satisfactory.