Two aircraft with similar sounding call signs on the same frequency can dramatically complicate communications. Similar sounding call signs can happen at any airport, but the probability increases at airports where an airline has a hub or where a flight school has several aircraft whose call signs only differ by a digit or two. Listen carefully to the frequency chatter and be alert for any other aircraft that has a call sign that is similar to yours. NXX67H taxied out to a hold short position for Runway 13 via Taxiway A. While NXX67H was holding short, Tower Control (TC) was controlling several other aircraft including NYY67H. Using only 67H as the call sign, TC cleared NYY67H for departure and subsequently for a turn on course and frequency change. A transmission (possibly from NYY67H) was partially blocked but the final portion was heard as ...67H, right turn westbound. Following this transmission, NXX67H assuming the instruction was intended for them, entered Runway 13 and departed without contacting TC. NXX67H’s departure conflicted with Cessna C152 on 1/8 mile final. The Cessna 152 saw NXX67H, initiated a go-around, and then side-stepped to avoid over flying NXX67H.5. The Cessna 152 saw NXX67H, initiated a go-around, and then side-stepped to avoid over flying NXX67H. Here, initiate means .