Abstract <p>Photoimmunotherapy has recently emerged as a promising method of treating cancer. It is based on selective tumor delivery of light-activated cytotoxic agents, or photosensitizers, by conjugating them to antibodies against tumor-associated antigens, creating photoimmunoconjugates. Despite the promising preclinical performance of several such conjugates, only those based on IR700DX (also known as IR700), a hydrophilic silicon phthalocyanine, have entered clinical trials. Thus far, this has resulted in one clinical approval. However, not all of the numerous reported IR700DX-based immunoconjugates demonstrate impressive activity <i>in vivo</i>, even when the antigen is a validated target for antibody–drug conjugates (ADCs). Importantly, the principal mechanism of action of these conjugates is unique among photosensitizers, which impacts their activity profile. This review surveys all IR700-based conjugates disclosed to date, seeking to elucidate the factors underpinning their therapeutic efficacy. The review also offers a perspective on possible future avenues of research concerning these promising targeted phototherapeutics.</p>