Superconductivity and ferromagnetism are antagonistic types of ordering, and their mutual effects give rise to several interesting phenomena which have recently been studied in rare earth compounds. A theoretical analysis shows that while a ferromagnetic superconductor is a type II superconductor near the superconducting transition point $T_\mathrm{c1}$, it becomes a type I superconductor near the ferromagnetic transition point $T_\mathrm{M}$. A new theory derived for the case $T_\mathrm{M}\ll T_\mathrm{c1}$ predicts the formation of a transverse domain-like magnetic structure near $T_\mathrm{M}$. In clean superconductors the electron spectrum is gapless. A change in the behavior from type II to type I upon cooling to TM has been observed experimentally in ErRh$_4$B$_4$. Experimental data on ErRh$_4$B$_4$, HoMo$_6$Se$_8$ and HoMo$_6$Se$_8$ prove the existence of superconductivity and a magnetic ordering below $T_\mathrm{M}$.