**1. Introduction**

In women, breast cancer is a prevalent cause of cancer worldwide [1]. It affected 2.3 million women globally in 2020, with 685,000 deaths. It has been diagnosed in 7.8 million women in the past 5 years, making it the most common type of cancer in the world [1]. Although breast cancer diagnostic methods and therapeutic procedures have improved in the past decade, the long-term survival of these patients remains low due to a high rate of postsurgical relapse. The efficacy of breast cancer treatment is limited by drug toxicity, multidrug resistance, and a lack of definitive prognostic biomarkers [2]. Thus, there is an urgent need to develop novel biomarkers and therapeutics to cure the disease.

In recent years, many studies have suggested that intercellular communication plays a key role in driving various cellular functions and homeostasis in physiological as well as pathological conditions such as cancer, cardiovascular diseases, and neurological disorders. Cancer development is mainly dependent on interactions between cancerous cells and their microenvironment components. Some of these interactions are mediated by extracellular vesicles, which alter the phenotype of recipient cells [3–5].

Extracellular vesicles (EVs) are spherical nanoparticles shed by all types of cells, including archaea, prokaryotes, eukaryotes, and fungi in the extracellular milieu [6]. These typically range from 30 nm to 5 μm in diameter based on their type and vary widely in composition [7]. In addition to being released during disease pathology, EVs allow various cells to send and receive messages to crosstalk with other cells, thus carrying out various biological functions [7]. These are mainly composed of different proteins, lipids, nucleic acids, and enzymes [8]. EVs circulate through many body fluids, such as blood, serum, and urine. Owing to their structural similarity to the parental source, they are considered potential biomarkers for diseases such as cancer [9]. To study the characteristics and functions of EVs, they are isolated using different techniques such as differential ultracentrifugation, size-exclusion, and ultrafiltration [10].

EVs are generally categorized into exosomes, microvesicles, and apoptotic bodies according to their release mechanism, size, and composition [3]. Exosomes are 30–150 nm in diameter and are formed by inward budding of the plasma membrane of the cell [9]. Microvesicles are formed by direct outward budding of the cell's plasma membrane and range in size from 100 to 1000 nm in diameter. Consequently, they are reported to contain mainly cytosolic and plasma membrane proteins, such as tetraspanins. Apoptotic bodies are shed during cell death into the extracellular space, ranging from 50 to 5000 nm in diameter. These generally contain intact organelles, glycosylated proteins, and chromatin, unlike the other two types of EVs. Among these, exosomes have been widely studied since their role in intercellular communication has been reported. This chapter will focus on exosomes and their potential applications as therapeutics for breast cancer.
