The present disclosure provides a method for monitoring residual disease in a subject. Generally, the method comprises determining a frequency of cancer mutations from a first sample obtained from a tumor biopsy from the subject; determining a frequency of the cancer mutations discovered in the first sample from a second sample comprising cell-free deoxyribonucleic acid (cfDNA) molecules that is obtained from the subject after the subject has undergone a course of treatment for cancer; and determining a presence or absence of cancer in the subject based on an analysis of the frequency of the cancer mutations from sequence data from the second sample.