The AMPLATZER Muscular VSD Occluder was approved by the US Food and Drug Administration (FDA) in September, 2007. This study is designed to further evaluate the safety and effectiveness in subjects implanted with the AMPLATZER Muscular VSD Occluder.

Inclusion Criteria:

1. complex congenital muscular ventricular septal defects (VSD) of significant size to warrant closure. Subjects must meet at least one of the following:

   1. Large volume left to right shunt (Qp/Qs >2:1),
   2. Pulmonary hypertension (PA pressure >50% systemic) and/or
   3. Clinical symptoms of congestive heart failure
   4. Banding of the pulmonary artery(ies) and in the opinion of the treating investigator closure of the VSD is clinically warranted.

2. Subjects must meet at least one of the following criteria to be considered high risk for standard transatrial or transarterial surgical closure based on anatomical conditions:

   1. Left ventriculotomy or an extensive right ventriculotomy,
   2. Failed previous VSD closure,
   3. Multiple apical and/or anterior muscular VSDs ("Swiss Cheese Septum"), and/or
   4. Posterior apical VSDs covered by trabeculae.
   5. Overall medical condition

3. Subject/legally authorized representative has signed the informed consent

4. Subject/legally authorized representative is willing to complete the follow-up requirements of this study

Exclusion Criteria:

1. Subjects with defects less than 4 mm distance from the semilunar (aortic and pulmonary) or atrioventricular valves (mitral and tricuspid)
2. Subjects with severely increased pulmonary vascular resistance above 7 Wood units and a right-to-left shunt and documented irreversible pulmonary vascular disease
3. Subjects with perimembranous (close to the aortic valve) VSD
4. Subjects with post-infarction VSD
5. Subjects who weigh < 5.2 kg
6. Subjects with sepsis (local/generalized)
7. Subjects with active bacterial infections
8. Subjects with contraindications to anti-platelets