Large amplitude Alfvén waves in the interplanetary medium are studied using data from Mariner V (Venus 1967). These waves, which propagate outward from the Sun, dominate microscale structure at least 50% of the time, with energy densities comparable to the magnetic and thermal fields. They are most prominent in high-velocity solar wind streams and their trailing edges, where velocity decreases slowly. In low-velocity regions, Alfvén waves are present but with smaller amplitudes and more mixed with non-Alfvénic structures. The largest amplitude Alfvénic fluctuations occur in compression regions at the leading edges of high-velocity streams, where velocity increases rapidly. These regions may contain inwardly propagating or non-Alfvénic wave modes. Power spectra of the interplanetary magnetic field show frequency dependencies from f^-1.5 to f^-2.2, with slower fall-offs associated with higher temperature regions. The microscale magnetic field fluctuations exhibit a 5:4:1 power anisotropy in an orthogonal coordinate system, strongest in compression regions at the leading edges of high-velocity streams. Magnetoacoustic wave modes are not identified, suggesting strong damping. The study organizes observations based on a model of solar wind velocity structure. Alfvén waves are likely undamped remnants of waves generated near the Sun. High wave activity in high-velocity, high-temperature streams suggests extensive heating by wave damping near the Sun. The highest Alfvénic wave activity in compression regions may result from amplified ambient Alfvén waves or fresh wave generation from stream collisions. The absence of magnetoacoustic modes indicates strong damping. The observed anisotropy is due to the partial conversion of Alfvén waves to damped magnetoacoustic modes as they convect away from the Sun. Alfvén waves are dynamic, non-shock structures, often non-sinusoidal and non-periodic, propagating in the rest frame of the solar wind. Vector correlations between magnetic field and bulk velocity support the presence of Alfvén waves. The correlation between b and v is strong, with variations in b comparable to field strength. The scale ratio used for plotting corresponds to D_A^-1 of 6.4 km sec^-1/gamma. Alfvén waves are observed in high-velocity streams and their trailing edges, with lower amplitudes in low-velocity regions. The frequency range of Alfvén waves is extremely broad, extending from 10^3 km to 5 × 10^6 km. Cross-spectra of the magnetic field and velocity show high coherency and 0° phase, indicating Alfvénic waves. The presence of Alfvén waves is common, with pure examples found in high-velocity streams and their trailing edges. The amplitude and direction of propagation are influenced by macroscale velocity structure, with outwardly propagating AlfvLarge amplitude Alfvén waves in the interplanetary medium are studied using data from Mariner V (Venus 1967). These waves, which propagate outward from the Sun, dominate microscale structure at least 50% of the time, with energy densities comparable to the magnetic and thermal fields. They are most prominent in high-velocity solar wind streams and their trailing edges, where velocity decreases slowly. In low-velocity regions, Alfvén waves are present but with smaller amplitudes and more mixed with non-Alfvénic structures. The largest amplitude Alfvénic fluctuations occur in compression regions at the leading edges of high-velocity streams, where velocity increases rapidly. These regions may contain inwardly propagating or non-Alfvénic wave modes. Power spectra of the interplanetary magnetic field show frequency dependencies from f^-1.5 to f^-2.2, with slower fall-offs associated with higher temperature regions. The microscale magnetic field fluctuations exhibit a 5:4:1 power anisotropy in an orthogonal coordinate system, strongest in compression regions at the leading edges of high-velocity streams. Magnetoacoustic wave modes are not identified, suggesting strong damping. The study organizes observations based on a model of solar wind velocity structure. Alfvén waves are likely undamped remnants of waves generated near the Sun. High wave activity in high-velocity, high-temperature streams suggests extensive heating by wave damping near the Sun. The highest Alfvénic wave activity in compression regions may result from amplified ambient Alfvén waves or fresh wave generation from stream collisions. The absence of magnetoacoustic modes indicates strong damping. The observed anisotropy is due to the partial conversion of Alfvén waves to damped magnetoacoustic modes as they convect away from the Sun. Alfvén waves are dynamic, non-shock structures, often non-sinusoidal and non-periodic, propagating in the rest frame of the solar wind. Vector correlations between magnetic field and bulk velocity support the presence of Alfvén waves. The correlation between b and v is strong, with variations in b comparable to field strength. The scale ratio used for plotting corresponds to D_A^-1 of 6.4 km sec^-1/gamma. Alfvén waves are observed in high-velocity streams and their trailing edges, with lower amplitudes in low-velocity regions. The frequency range of Alfvén waves is extremely broad, extending from 10^3 km to 5 × 10^6 km. Cross-spectra of the magnetic field and velocity show high coherency and 0° phase, indicating Alfvénic waves. The presence of Alfvén waves is common, with pure examples found in high-velocity streams and their trailing edges. The amplitude and direction of propagation are influenced by macroscale velocity structure, with outwardly propagating Alfv