Loneliness, the subjective distress that arises when a person\'s social relationships are perceived as deficient in quantity or quality, has emerged as a significant public health concern among young adults. This paper reviews the theoretical foundations of loneliness research, including Peplau and Perlman\'s cognitive discrepancy model, Weiss\'s typology distinguishing emotional from social loneliness, and Cacioppo\'s evolutionary and neurocognitive account of loneliness as a social-threat signal. The review synthesizes empirical evidence indicating that loneliness peaks during emerging adulthood relative to other life stages, and that loneliness levels among young adults have risen over recent decades according to cross-temporal meta-analytic evidence. Particular attention is given to the relationship between social media use and loneliness, where meta-analytic findings indicate that problematic and compulsive use patterns are more consistently associated with loneliness than overall time spent online or the simple distinction between active and passive use. The review then considers loneliness among young adults in India, drawing on systematic review evidence documenting a substantial burden of loneliness across Indian populations and recent empirical work linking meaning in life and thriving to reduced loneliness and enhanced well-being among urban Indian youth. The review concludes by identifying gaps in the literature, including the need for longitudinal and culturally grounded research beyond urban and student populations, and proposes directions for future research.