This document discusses best practices for defining and measuring latency service level objectives (SLOs). It recommends computing SLOs directly from raw log data using histograms, which allow arbitrary percentiles to be derived and are better than averaging sample percentiles. Histograms can also be aggregated over time and used to count the number of requests above a latency threshold regardless of what the threshold was set to initially. Common histogram implementations like HDR-Histogram and t-digest are suggested.