Given the pervasive role of smartphones in modern life, research into their impact on well-being has flourished. This study addresses existing methodological shortcomings using smartphone log data and experience sampling methods (ESM) to explore the bidirectional within-person relationship between smartphone usage and momentary well-being variables (i.e., affect valence, loneliness, positive affect, and negative affect). We further examine different categories of smartphone usage, namely, communication, social media, and other apps. We analyze three samples (