• Save the spreadsheet as CSV (comma-separated values), not as an Excel file. The importer tells you if you have picked an Excel workbook.
• The first row must be headings, with at least one row under it.
• On the Match the columns step, check each field points at the right column of your file. A contact needs a name or an email.
• Rows whose email is already in your account are skipped unless you choose Update records that are already here.
• Up to 5,000 rows and 8 MB a file. Split larger files.

After importing, the last page lists every skipped row with the reason.