You do not strictly need to book a Shinkansen (bullet train) ticket in advance, as most trains offer Non-Reserved (Jidoseki) cars where you can simply board with a basic ticket and find any open seat. However, for peace of mind and specific routes, booking is highly recommended. During peak travel seasons like Golden Week, Obon, or New Year’s, trains sell out quickly, and standing in the aisle for hours is common if you haven't reserved a seat. Additionally, as of 2020, passengers carrying "oversized baggage" (total dimensions between 160 cm and 250 cm) are legally required to book a specific "baggage reservation" seat in advance. If you fail to do this, you may be charged a 1,000-yen fine and forced to move your luggage. For the Tokaido, Sanyo, and Kyushu lines, using the SmartEX app or visiting a JR Ticket Office (Midori-no-Madoguchi) a day or two before your trip is the safest bet to ensure a comfortable journey.