  We discuss quantum phase transitions in the pseudogap Kondo problem, which
describes a magnetic moment coupled to conduction electrons with a power-law
density of states, rho(omega) ~ |omega|^r. We show that different perturbative
expansions, together with renormalization group techniques, provide effective
low-energy field theories for the relevant critical fixed points. In
particular, we review expansions near the lower-critical and upper-critical
dimensions of the problem, being r=0 and r=1, respectively.
