Tumor-associated macrophages (TAMs) are the most prevalent immune cell in the tumor microenvironment. Their high abundance in tumors has been strongly correlated with poor prognosis and patient survival across many cancer types. Despite their exhibition of immune stimulating M1-like phenotypes at an early stage of cancer development, TAMs mainly adopt an immunosuppressive M2-like phenotype. Extensive evidence has shown that M2-like TAMs support cancer cell survival, dampen cytotoxic adaptive immunity, promote angiogenesis, and facilitate tumor metastasis. Because of their negative influence in almost every step of tumorigenesis, targeting M2-like TAMs represents an emerging anti-cancer therapeutic strategy. However, our ability to exploit TAMs therapeutically has been stymied by two challenges: 1) an incomplete understanding of targetable pro-tumorigenic pathways and 2) limited knowledge of the mechanisms producing their M2-like phenotype. Overcoming these two challenges is required to develop effective TAM-targeting therapeutics and identify patients that might benefit from them. Studies in Chapter 2 (aiming to tackle the first challenge) not only identified lysosomal cysteine protease activity in TAMs as an important immune checkpoint in regulating antigen cross-presentation in cancer but further described a DNA nanodevice that can be targeted with organelle-level precision to alleviate this checkpoint in TAMs and achieve immunomodulation in vivo. Studies in Chapter 3 (aiming to tackle the second challenge) not only demonstrated a metabolic pathway to promote M2-like phenotypes in TAMs through a novel epigenetic mechanism, but also provided a rationale for repurposing LDHA inhibitors to target TAMs using the lactate-Kla-M2 pathway as a biomarker. Collectively, our studies clarify mechanisms contributing to the pro-tumorigenic functions of TAMs and seeded an approach to therapeutically target them.