SMS segments are the billing units for text messages. Longer messages are split into multiple segments, each billed separately. Segment limits: - Standard (GSM-7): 160 characters - Unicode (emojis, special chars): 70 characters - Concatenated GSM-7: 153 chars/segment - Concatenated Unicode: 67 chars/segment Cost implications: - 161-306 characters = 2 segments - Emojis reduce character limit significantly - MMS has different pricing Best practices: - Keep messages under 160 characters when possible - Use link shorteners - Be mindful of emoji impact - Track segment usage in analytics