Although understanding of the molecular biology of cancer has advanced and medicine has an impressive arsenal of chemotherapeutic drugs, the problem of tumor resistance to individual drugs and drug combinations has not yet been resolved. Known mechanisms of cancer chemoresistance do not explain the reason for such a phenomenon as “apoptosis-induced proliferation,” where cells dying under the effect of the therapy secrete some signaling molecules into the extracellular medium to promote proliferation, survival, and acquisition of a more aggressive phenotype of neighboring cancer cells. The nature of this unexpected phenomenon is only now beginning to be partially clarified, but the nature of such signaling between apoptotic cancer cells and their neighboring cells remains largely unknown. For this reason, in this review we discuss currently known types of intercellular communication of tumor cells, give specific examples of important secreted molecules involved in signaling between cancer cells, and describe possible cell interactions contributing to the progress of cancer outgrowth.