Three aspects of magnetic white dwarf stars are studied to aid in the understanding of stellar evolution. A survey of ~ 50 DC white dwarf stars was conducted in circular spectropolarimetry to search for magnetic fields >~ 30 kG. Four DC stars were discovered with magnetic fields above 30 kG: G 111-49 with Be ~ -220 MG, G 183-35 with Be= +6.8 ± 0.5 MG, G 256-7 with Be= +4.9 ± 0.5 MG, and G 234-4 with Be=+39.6 ± 11.6 kG. A new magnetic DB white dwarf was also discovered, LB 8827 with Be= 1.0 ± 0.5 MG. A total of 15% of the white dwarfs in the survey have a magnetic field > 30 kG. This value is far larger than the 2% of DA stars, but more than half of the DC stars were originally misclassified. Only 5% of the re-classified DC stars have magnetic fields above 30 kG. Three magnetic stars from the DC white dwarf survey were re-observed to investigate the possibility of rotation. Two are definitely rotating: LHS 1734 with 16 min <~ P < 1 yr and G 158-45 (=LHS 1044) with a probable period P ~ 4.44hr but a definite period P The isolated magnetic white dwarfs G 99-47, KUV 813-14 (KUV 23162-1220), and G 227-35 were observed in linear and circular spectropolarimetry and then compared to calculated theoretical spectra to find a model for the magnetic field strength and structure. The comparisons were to Stokes' V/I (circular polarization) spectra in addition to total flux Flambda, and these add many constraints to the possible solutions. An off-centered dipole or a dipole+quadrupole configuration best fits the observations. The results of the survey and the modeling are consistent with the theory that the magnetic Ap stars are the predecessors of magnetic white dwarfs.